US2378321A - Electrical silicon steel - Google Patents

Description

Patented June 12,1945

' OFFICE 2,318,321 ELECTRICAL SILICON STEEL Matti H. Pakkala, Vandergrift, Pa.

No Drawing. Application January 6, 1943, Serial No. 471,475

2 Claims. (01. 1 18-12) This invention relates to methods for producin electrical silicon steel flat stock and to the products resulting therefrom.

More specifically, this invention is concerned with the production of magnetic material characterized by watt loss values regarded as suitable in reference to the standard grades for such products, yet which involves metallurgical departures from steels usually associated with such grades by being lower in silicon content, higher in carbon content and by being reduced by cold rolling to gauge from hot strip sizes without impairment of its isotropic properties.

Also, electrical steels made according to this conventional practices in that the carbon content, recognized-to be deleterious in its effects upon the magnetic properties 01' watt loss and permeability, may need be no lower than those values usually identified with low steels, thus enabling many economic benefits to be derived from the steel making end, without adversely influencing the electrical properties.

Secondly,'the electrical steels herein contemplated differ from the recognized standard grades of such steels by presenting equal electrical chanical properties, as imparted by the concluding treatments of my method, so as to be substantially non-aging, and, thus, distinct from mild carbon steels, containing silicon or otherwise, which are drastically cold reduced to gauge.

Fourthly, the present invention is applicable to all grades of electrical silicon steel, whether aluminized or not aluminized, ranging from the lowest commercial grades containing around 0.10%, up to the highest grades containing up to 5.0%, silicon, thus presenting one simplified rule of processing for all types of electrical steels to replace the heterogeneity of processes of varying degrees of complexity which characterizes the present state of the art.

Several discoveries assisted in the formulation of my improved processing about to be described. Among these is the fact that in a silicon steel sheet or strip which has been severely cold reduced, a marked directionality in the direction of cold rolling is most pronounced when the material has the larger grain size, such as, No.

' 2 to No. 1 grain size of the A. S. T. M. standard,

properties with a minimized silicon content, in a the higher grades, which appear one percent or more lower than is usual for such grade. This is responsible for lowered initial costs, and for greater facility in processing, especially with reference to cold reduction, and lowered heat treating temperatures. I

Thirdly, the present invention departs from the established treatments and concepts by providing for the production of silicon steel strip and sheets by cold reduction from final hot-strip gauges without establishing in the metal more favorable electrical properties in one direction (e. g., the direction of rolling) than in another direction normal .to the first, and is thus free from substantial directionality in favor of good isotropic properties. Further, my steel is marked by a permanence of electrical and meor larger. On the other hand, improved isotropic characteristics are achieved by the development of uniformly intermediate size grains on the order of No. 3 to No. 6, inclusive, of the A. S. T. M. standard, for the same degree of cold reduction.

This optimum condition of crystallinity is considerably influenced by the, disposition of the carbides in the ferrite. If the carbides upon cooling of the metal remain unagglomerated, or'

are allowed to form in a fine lamellar pearlite structure, they somehow provide an effective obstruction to grain growth. When they coalesce,

however, they cease to be as effective as grain Therefore, the carbides in the iron should be precipitated and agglomerated into massive carbidic and/or pearlitic concentrations, rather than in a fine pearlitic banded structure.- In this condition, they do not interfere with the proper grain development during annealing, nor form unstable relationships provident of the degeneration.

of the magnetic and mechanical properties upon aging.

Thus, in giving efiect to the present invention,

one is governed by the principle that the amount of carbon present in the steel is less important than its disposition in the final product. As will appear in specific examples and results set forth hereinafter, the carbon content may range up to 0.05% in any grade of silicon steel selected for processing hereunder, without adversely afiecting the electrical properties, while equaling or bettering those steels of the prior art where, for comparable results, the allowable carbon could not substantially exceed 0.01%. As to the mechanical properties, the carbon lends additional stiffness thereby to improve the punching'properties of the steel, and is more helpful than harmful in this respect.

Everything that has been mentioned in reference to the carbon content, applies equally to the segregates. and other inclusions in the metal, all of which, together with the carbides, should be well agglomerated at the grain boundaries. In view of this, it is desirable, in the interests of minimizing the heat treatments to follow, to select a steel which comes from the hot-mill with its carbides (including coarse pearlite, if pearlite is present) and segregates well agglomerated, such as strip that has been delivered from the finishing stand of the hot-mill at high temperature (1700 F. more or less) and coiled hot (1200 to 1400 F.) so as to maintain conditions tending toward equilibrium in the ensuing cooling to room temperature.

During the subsequent cold rolling to gauge, or nearly to gauge, such agglomerates resist fragmentation, and are not dispersed into stringers or bands, such as characterize fine pearlitic structures, which interfere with the proper grain growth. By the preestablishment and preservation of this agglomerated condition, the processing by heat treatment after cold reduction may be greatly simplified and expedited. Therefore, even though not absolutely necessary, in that it can be acquired by critical and prolonged heat treatment after hot or cold reduction, a well agglomerated structure off of the hot-mill is decidedly preferred.

The hot-rolled material is then pickled, cold reduced to gauge, and heat treated, in one of two ways, which alternatively invoke the same fundamental principle, while adding flexibility and convenience to the application thereof. The first of these involves cold rolling to within 3% to 8% of gauge, annealing at low temperature, temper rolling to gauge, and finally annealing again at low temperature to recrystallize. The other involves simply'cold rolling to gauge and annealing at relatively higher temperatures.

The condition of the metal, as to its ferritic character and as to the agglomerates, having been established, the considerations to be weighed against the preservation of this condition are those of strain versus time-temperature-mass, looking toward the development of optimum crystallinity, as mentioned before.

Of the first importance in this connection is the selection of a heat treating schedule which will avoid those temperatures at which the steel becomes predominately austenitic, since the agglomerated carbides, being as they are substantially insoluble in alpha iron (ferrite) are readily soluble in gamma iron to form austenite, which, in predominant amounts destroys one of the fundamental conditions incident to the successful practicing of the invention. This will be better appreciated in view of the intermediate grain size already stated as being desirable which it is the purpose of this anneal to attain. If the temperatures are such as to render the steel predominantly austenitic, it will, upon cooling, provide a much finer grain structure as to the ferrite crystals, even if the carbidic and pearlitic masses are well coalesced. This is because austenite represents a second phase which must transform to ferrite and carbide upon being cooled and, for the reason stated, the resulting ferrite will have a much smaller grain size than it was the purpose of such an anneal to provide. Thus, it is desirable to avoid those treatments which, if applied, would deprive the steel of its predominantly ferritic nature. This means the selection of a heat treatment which will, in relation to the chemical constituency of the metal, skirt the socalled gamma loop so as to preclude the formation of gamma iron, which is the austenitic form.

Chemically, the basic influences entering into the ferrite-austenite relationship are those of the carbon and silicon, and also aluminum, where this last element is present. Generally speaking, the temperature at which austenite is formed increases as the silicon content increases, but decreases as the carbon content increases. Thus, a mild steel or low carbon iron containing carbon within the range 0.01 to 0.02 per cent, in the total absence of silicon (and aluminum), will become austenitic at about 1665 F., while at 1.0 per cent silicon this will become increased to above 1750 F. At about 2.2 per cent silicon, steel of this carbon range does not become entirely austenitic at any temperature, and at approximately 2.5 per cent silicon or over, no austenite is formed.

In the higher carbon steels, containing carbon from about 0.05 to 0.08 per cent, approximately 1335 F. is the temperature at which austenite begins to form at zero silicon content, which increases as the silicon content increases so as to be not entirely austenitic at any temperature at about 2.3 per cent silicon. At 4.0 per cent silicon, this temperature, (i. e., at which austenite begins to form) will have become approximately 1480 F. The silicon content above which such steel will remain entirely ferritic irrespective of temperature has not been determined, but is known to be considerably over 4.0 per cent silicon. Commercial alloys containing 0.02-0.03 per cent carbon, and 4.4-4.8 per cent silicon have been found to be ferritic at all temperatures.

The efiect of aluminum in alloying amounts, 0.25 per cent being representative in the final analysis, is to reduce the area of the gamma loop, and, thus, to raise the temperatures to which the steel may be raised without forming austenite. Aluminum also has the effect (in substantial alloying amounts as here contemplated) of causing the development of large uniform grains at temperatures considerably below those necessary for the attainment of a corresponding grain size in non-aluminized silicon steels. Whereas, the temperature at which a 0.25 per cent aluminumbearing silicon steel becomes predominately austenitic may vary somewhat as the silicon content varies, a satisfactory condition seems to be attained at 1650 F. for steels containing 2% or more silicon with reference to a final grain of intermediate size. The practical effect of the aluminum is, therefore, to lower the effective temperatures at which the higher silicon steels must be treated for the attainment of the proper crystalline condition without incurring the risk of intersecting the gamma loop in the higher carbon tent (if any), and its relation to the economics of the commercial picture must be weighed. Thus, in the lowest electrical grades, involving steels containing 1% silicon or less, and in which there is no aluminum present, the carbon content in the order of .04-.05 per cent would enter the austenitlc phase upon being elevated to approximately 1450-1525 F. This class of material is preferably processed by the so-called double treatment, in which it is cold reduced from hot strip sizes to within 3-8 per cent of gauge and annealed at temperatures around 1500 or less, at which the steel remains predominantly ferritic. A single anneal at these temperatures is, however, insuflicient to develop the intermediate grain. size herein disclosed as being necessary for the attainment of isotropic properties. To achieve this desideratum,

it is necessary to impart the requisite strain to.

the material by temper rolling to gauge, followed by an anneal at or below the temperatures adopted for the first anneal for the purposes of attaining the requisite grain growth. In the case of each anneal, the material is slowly cooled from temperature in order to' preserve the conditions of equilibrium so as to insure the reprecipitation and reagglomeration of such of the carbides and segregates as may have been redissolved during the heat treatment. For reasons already set forth herein, the maximum magnetic properties are developed by this condition in the metal.

Since, as has been noted above, the temperatures necessary for the attainment of the gamma phase increase as the carbon content decreases, or. alternatively, increase as the aluminum content increases when there is aluminum present, the low grades, above discussed could be treated by a single higher temperature anneal provided the carbon content were adjusted downwardly or an aluminum content established in the metal to allow for the attainment of the proper grain size without sacrificing the ferritic nature of the steel. However, since the cost factor of producing the lower grades is a considerable item, it is considered desirable to adopt the double treatment involving the temper roll, as above described, rather than the more costly practices entailed in radical decarburization in the open-hearth, or in providing an aluminum alloy steel.

Based on these economic considerations, the herein-described double treatment will be effectively applied up to the point where the silicon content has suificiently raised the austenite for mation temperature to admit of the attainment of the proper grain size by a single anneal applied after cold reduction from the hot strip mill final size to finalgauge. Since, within the silicon range of 2.05.0 per cent, the effective heat treating temperatures for the attainment of the optimum magnetic properties increase in direct proportion with the amount oi silicon present, it becomes an economic expediency to add aluminum in order to attain the desired metallographic condition of the metal under a time-temperature condition substantially lower than would be the case in the absence of aluminum. Even though at these higher silicon and aluminum contents the metal remains predominantly ferritic at all temperatures, it is essential to conform to good economic practice to minimize the heat treating temperatures for obvious reasons that need no elaboration; one of which is the fact that a continuous annealing may be applied satisfactorily in lieu of the slower box annealing methods.

It is significant that continuous annealing, whether applied by itself, or in conjunction with box annealing (it does not matter whether the continuous anneal is first or second, in relation to the box anneal) seems to increase the permeability of the steel materially, and, for this reason,

- may often be applied in preference to box annealing. The reason for such improvement is not understood, although, it has been surmised that the tension of the work-piece by its own weight at temperature, as in a continuous catenary furnace, might play a part in this reasult.

Other economic considerations should be mentioned. Box annealing is a suitable method for the single annealing or double annealing of sheets or coils where the economical consideration are best served thereby. The lower grades (under 2% Si) with no aluminum content, if given the double treatment previously mentioned, need not have temperatures exceeding 1500 F. applied during either anneal, and lower temperatures are frequently admissible. Whether in coil form, or as sheets stacked in piles, no separator medium or coating is necessary to prevent sticking at these temperatures. Therefore, for the lower grades, the low temperature anneals are simple and economical.

The single anneal embodiment of the present invention, if applied as a box anneal, must be run at higher temperatures than the double box anneal mentioned above. Excepting aluminized silicon steels, sheets or strips processed in this manner must have the sticker-prevention coating, and other operating considerations must be observed which render this practice undesirable, in some instances, from the economic standpoint. Aluminized silicon steels need not be heat-treated beyond 1650 F., rendering the application of a separator medium thereto unnecessary in most cases. In this connection, it will be realized that the aluminized grades are usually those containing the higher silicon contents, as has already been explained. The higher the silicon content,

the higher may be the heat treating temperatures applied without sticking in the absence of a separator coating.

Continuous annealing, applied singly, or doubly.

with itself or witha box anneal, has much to recommend it for the simplicity of control and great expedition with which strips or sheets may be processed. Its limitations are that, when applied as a single anneal, as herein contemplated, the time-temperature factor cannot be made sufliciently high to anneal other than the lower grades (e. g., 2% silicon or less), unless, as has already been mentioned, aluminum, say, in the order of 0.25%, is contained in the steel. It is admirably adapted for continuously annealing strips and sheets, in accordancewith the shorter method herein disclosed, which have 2% or less silicon content, or which contain aluminum, as above stated. Obviously, the time-temperature factor of continuous annealing may be adapted to the higher silicon, non-aluminized grades, by passing such material more than once through the furnace.

It is also adapted to working inconjunction with a box anneal, in the double treatment, or with another continuous anneal. In either case, the time-temperature factor is not so high a value as when the single treatment is utilized, rendering the application of continuous this manner highly practicable.

annealing in Governed by the economical consideration and operating problems discussed in the foregoing paragraphs either the single treatment or the double treatment is elected, and carried out as follows: I

First, the double treatment. The material, either strip or sheets, but preferably in strip form, is cold-reduced to within temper pass or gauge, the extent of the temper roll allowance being determined by considerations presently .to be discussed. After reduction, the mtaerial is annealed, either continuously or in bulk, at temperatures (depending upon the slicon and aluminum contents, if aluminum is present) at around 1400" F. (for 2% Si or less-no aluminum), up to 1650 F. for the aluminized grades, and higher (1950 to 2200 F.) for the non-aluminized high silicon grades. Generally, however, since the double treatment is more applicable to the inferior grades, as has already been explained, the treatment will be ordinarily around 1400" F. for hours in the case of a box anneal, or at 1850 F. for /2 to 6 minutes in a continuous anneal. In any case, this anneal is followed by a slow cool.

The temper rolling to gauge is employed to strain the material sufficiently so that, upon application of the final heat treatment, the straintemperature grains to the intermediate sizes preferred for optimum isotropy, as previously discussed, but will not be within the germinative range. Germinative grain growth, if realized, would provide relationship will be sufficient to growsuch coarsening of the grains as to impair the isotropic properties here contemplated. An extension of 3% by temper rolling, followed by an annealing at about 1325 F. for eight hours, may be considered representative.

The final heat treatment is, thus regulated by the amount of cold strain imparted by the temper pass, and, in general, varies inversely as the latter. A large amount of strain (say, 5% or more elongation) would necessitate a low temperature anneal (i. e., low with regard to the silicon content), while a lesser degree of strain would admit of higher temperatures, without, in either case, encountering the germinative range.

Following this reasoning further, a very slight temper pass would require a relatively high temperature; finally, no temper pass would require the highest temperature, and would result quite logically, in the merger of the two lower temperature anneals, into a single high temperature anneal. Thus, is the single or short treatment derived through another approach.

The single anneal, or short treatment. Steel strip or sheets, having the carbides and segregates well precipitated and agglomerated, as previously described, are cold-reduced to gauge (1. e., continuously, from hot-strip sizes). Selected in light of the considerations discussed above, nonalumlnized grades containing up to 2% silicon may be continuously annealed at about 1850 F. until the intermediate grain size is attained. In box annealing this usually requires a temperature from 1500 to 1650 F. for 5 to 20 hours. The non-aluminized grades containing more than 2% silicon will be treated at accordingly higher temperatures, up to 2300 F. for the highest silicon contents (3.50 to 5.0% silicon), the temperature varying upwardly in proportion to the latter. For these treatments, box annealing. is preferred, since the time-temperature factor is necessarily so high as to render one continuous annealing step impracticable from the economic standpoint.

As to the aluminized grades (those containin about 0.25% residual aluminum), irrespective of the silicon content, a temperature of approximately 1650" F. is preferred, both in box annealing or continuous annealing, as has already been explained herein. At higher temperatures, uncontrolled grain growth, possible intersection of the gamma phase, and unavoidable oxidation of at least some of the aluminum content of the steel would lead to inferior results.

Irrespective of the type of heat treatment adopted, whether the single or double anneal is elected, or whenever the metal has been heated above its transformation point, e. g., during annealing or preparatory to, and during, hot reduction, the metal must be lowered to below its transformation point very slowly. This preserves conditions approaching equilibrium during cooling and effects the reprecipitation of those carbides and segregates that will have been redissolved at temperature in the ferrite, causing them to agglomerate in the manner hereinbefore described as being necessary.

Thus, not only will the metal be left in the best condition for the attainment of the optimum grain condition, previously discussed, imparting the desired isotropic characteristics, but the magnetic and mechanical properties will likewise be best served thereby, and will not change upon aging.

Although the discussion of the invention has, thus far, been addressed primarily to the production of optimum isotropic electrical properties in all grades of silicon steel by cold reduction from usual hot strip sizes (0.060 to 0.080 inch gauge thickness or greater) down to final gauge sizes (0.014 inch, more or less), the benefits hereof are not to be denied to those silicon steel sheets and strip which are brought to final gauge size by less drastic amounts of cold reduction. It is believed to be an obvious corollary to what has already been said, therefore, that the heattreating time-temperature factor in the single treatment embodiment may be of a lower order where the cold reduction is appreciably lessened. Such heat-treating factor may be said to vary more or less directly as the amount of cold reduction, all else being equal, insofar as this is consistent with the teachings herein set forth.

This result is to be expected, since, by minimizing the amount of cold work, grain fragmentation and distortion are likewise minimized, requiring less energy in the form of heat for the recovery of the metal to a condition of grain size and orientation more compatible with the realization of optimum properties. Similarly, the condition of strain in the metal is lessened, tending in the direction of those critical strain relationships which respond at lower temperatures to reform the grain into larger sizes of more uniform and regularly placed patterns.

Specifically to illustrate this modification of the invention, if gauge reduction on the hot mill is prolonged to sizes substantially thinner than 0.080 inch; e. g., 0.060 inch or less, the final annealing temperatures may be lowered from l650 to 1850 F. down to 1575" F. or lower, or at the same temperatures, the time factor may be correspondingly reduced. If the cold reduction required from 0.080 inch hot strip sizes is reduced by one-half by proportional hot mill reductions, effective treating temperatures of 1475 F. or lower (or again a shorter treating time at approximately the same temperatures) may be employed. "In any case, the improved magnetic and mechanical properties herein set forth will be realized from these greatly simplified practices, so long as the general considerations previously discussed are observed.

The following table carries specific examples of aluminized and non-aluminized silicon steels processed in accordance with the present invention. The examples have been selected to show the effects of box annealing and continuous annealing and the relation of the double and single treatments to the aluminized and non-aluminized grades. In each case, it should be understood that the electrical values given represent an average between the length values (those taken in the direction of rolling) and the transverse values (those taken normal to the direction of rolling) but that, in no case, do these length and transverse values show a difference of It is interesting to note that some material, made in accordance with the present invention, after cold rolling, but before heat treatment, has exhibited much more favorable properties in the direction of rolling than in the transverse direction which,

' after treatment, not onlyv has revealed the equalization of the two values, but in some cases, has actually shown such an improvement that the transverse values have been better than the length values in the final product.

treatments, it is more by way of a single modified treatment than a double treatment, since, if the first anneal could be made of sufilcient order of time as would be the case if it were a box anneal 5 instead of a continuous anneal, then the second annealing step indicated would not be necessary. Note that the material is not subjected to temperature rolling between these two anneals and thus distinguishes from the so-called double treatment.

or more higher than is here given.

Notwithstanding the severe cold reduction sustained by the steel, its length and transverse core loss values did not diifer by 5%, the value of .60

given in this column being an average.

From the foregoing it will be appreciated that the electrical properties developed by this invention are equal to those attained by the far more elaborate methods of the contemporaneous and prior art, and are achieved on steels, the silicon contents of which, are lower than is usually for s0 sary heretofore.

grades usually associated with such electrical values. This has been done without necessitating the drastic decarburization of the metal at the open hearth, which has been regarded so neces- Optimum isotropy has been No. 4 No. a No. 6 No. 7

No. 1 No. 2 No. 3

Steel analysis:

Si ..per cent.. A1 .do.... 0

0 do. Hot mill final size.inch Pickle Goldmill final size-inch. .0175

Annealing No. 1 con- W a w 1950 (4 hrs tin'uous 1400 (20 hrs.). 1850 (26/mln.) 1600(25'/min.). 1950(25/min.). so Temper roll (extensionion None N None Nona Nmm N Annealing No. 2 continuous atch ..F N N n 1600(25'Imin.) None Coreloss watts b-- 1.18 1.28; .60. Permeability 16,000 500 75 376 ing operation carried out upon steel of the same silicon grade only having aluminum present in alloying amounts.

Examples 4 and 5 represent identical processing of the same steel by continuous annealing methods, except for the fact that one was heat treated at 1850" F., while the other was heat treated at 1600 F. The latter temperature, as set forth in Example No. 5, establishes this to be the 'most desirable for continuous annealing .aluminized grades of this kind, as is evidenced by the watt loss values.

Example No. 6 reveals that one continuous anneal does not afford a sufficient time-temperature factor to develop the electrical properties of nonaluminized grades having intermediate silicon contents. The prolongation of the continuous anneal, as is represented by a second anneal in...

achieved in a severely cold-reduced body, while the mechanical properties are improved over corresponding grades of silicon steel as determined by the electrical properties.

These and many other benefits are realized from the exceedingly simple and inexpensive practices here set forth, which are established upon the one fundamental principal of maintaining the silicon steel at all times predominantly ferritic while heat-treating to procure an intermediate size grain free from the fine pearlitic formations that tend to band and interfere with the grain development. All logical methods for giving efiect to this, though departing from the letter of this specification, come within the spirit and intent of the invention, as is more concisely apprehended in and by the appended claims.

I claim:

1. A method comprising hot-reducing into strip electrical silicon steel having a carbon to silicon ratio permitting austenite to form at hotreducing temperatures, coiling this strip prior to loss of its hot-reducing heat suilicient to cause ing this cold-reduced strip to develop its magnetic and electrical properties, said treating being limited in temperature to prevent material loss of the carbide agglomerates.

2. A method comprising hot-reducing into strip electrical silicon steel having a carbon to silicon ratio permitting austenite to form at hotreducing temperatures, coiling this strip prior to loss of its hot-reducing heat sumcient to cause material loss of austenite, formed by said heat, allowing the strip to slow-cool due to the coiled form and thereby providing the strip with massive agglomerates of carbides that resist fragmentation during cold-reduction of the strip, cold-reducing the strip to produce cold-reduced strip and treating this cold-reduced strip to develop its magnetic and electrical properties, said treating being limited in temperature to prevent material loss of the carbide agglomerates and consisting solely in box-annealing the coldreduced strip at'temperatures adjusted to develop its magnetic and electrical properties without material loss of the carbide agglomerates.

MATH H. PAKKALA.