US2229642A - Process of making electrical sheets - Google Patents

Description

Patented Jan. 28, 1941 PATENT OFFICE PROCESS OF MAKING ELECTRICAL SHEETS Robert L. Davidson, Middletown, Ohio, and Carl C. Mahlie, Hammond, Ind., assignors to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application July 3, 1937, Serial No. 151,944

8 Claims.

This application is a continuation in part of our copending application Serial No. 548,256, filed July 1, 1931 and entitled Electrical sheets.

Our invention relates to electrical sheets of given cores loss values which will be more ductile than any such sheets ever produced before, which sheets will be free of loose scale, permit of easier punching and handling in the manufacture of electrical machinery and afford a greater safety factor when used in rotating electrical apparatus.

More specifically, however, without restriction thereto, our invention has to do with electrical sheets comprising .iron and silicon such as used in transformers, dynamos and the like, having core losses less than 0.8 watts per pound when tested at a magnetic induction of 10 kilogausses and 60 cycles under the standard test conditions specified by A. S. T. M. (American Society for Testing Materials) so-called Epstein method, Serial No. A-34-36.

It is well known in the art that alloy steel sheets for electrical purposes should have a high magnetic permeability and a low core loss to give the best results, and the manufacture of such sheets is well understood in the art. One serious disadvantage accompanying such high grade electrical properties with present manufacturing methods is a very pronounced brittleness which is of great disadvantage, resulting in rapid wear of dies, breaking of punchings, etc. If such brittle material is used in rotating electrical machines, extremely costly damage may result due to the inability of the brittle material to withstand the concomitant stresses in operation.

In prior practice where there has been a core loss value of, for instance, 0.8 watt per pound in high silicon steel sheets, there corresponds a ductility of 4.7 millimeters as measured by the Erichsen test, while a material of 0.55 core loss has a ductility of only 2.5 millimeters.

Our improved sheets will have a considerably better ductility while the good electrical properties will not be affected. The lowest limit of ductility of our improved sheets at various core loss values corresponds to the foirmula E 8 W-l. where E means average Erichsen value in millimeters, taken at least at five places of a sheet which are substantially equally spaced from each other across the width of the sheet and W means core loss in watts per pound at 60 cycles measured by the standard A. S. T. M. method, specification No. A-34-36. For instance, we have made many tons of high silicon steel sheet material having a core loss value (determined on specimens comprised of strips, '50% of which are cut transverse and 50% parallel to the direction of rolling, and tested without annealing subsequent to cutting the specimens from the annealed sheet), of 0.57 and an Erichsen value of 4.1, a ductility which to our knowledge has 5 never been obtained before, with a silicon steel sheet of like electrical properties. The advantages obtained by the increased ductility of our product are quite considerable, the most important of which being probably increased die life 10 in making punching for electrical equipment, reduced loss of steel occasioned by breakage, and the possibility of safely using material of higher electric properties in rotating electrical machines,

which hitherto would have been unsafe because 15 20 In annealing silicon steel or other alloy electrical sheets it is sometimes the practice to pass a protective gas into the annealing hood, for instance, hydrogen or nitrogen, or a mixture of both. The purpose of such protective gases has been to prevent the sheets from becoming oxidized during the annealing, and not for the purpose of decreasing brittleness, nor has any such effect been noted in the literature.

By exhaustive research we have found that contrary to the general belief, the presence of molecular hydrogen, although in small percentages, with the sheets during the annealing process, causes brittleness. We have found that if one merely uses commercial nitrogen gas during the annealing as the protective gas, brittle sheets will also be obtained for reasons which will be set forth. We have made the discovery that this brittleness resulting from a nitrogen anneal is caused by small percentages of hydrogen, or moisture, in the nitrogenous atmosphere. By elimination of moisture and hydrogen from the nitrogen used as a protective gas in annealing silicon steel sheets, we are able to produce electrical sheets of greater ductility at a given core loss value than have ever been produced before. Prior uses of nitrogen in annealing for preventing development of oxide undoubtedly failed to eliminate contamination of the nitrogen gas, and hence no observable decrease in brittleness has been noted in the literature.

But we have also found that it is not suflicient merely to describe the composition of, the gas beingv passed into the annealing chamber. Our ex-.

periments have shown that a correct atmosphere must be maintained within the annealing box throughout a major portion of the anneal if material having improved ductility is to result. Certain gases are evolved from the charge of steel and from the furnace walls which must be removed or corrected.

Thus we have found that it is not sufilcient I merely'to start with a purified nitrogen gas or to take an impure nitrogen gas and purify it, afterward merely feeding the correct atmosphere into the annealing chamber. Theoretically, of course, if a purified atmosphere were furnished and were swept through the annealing chamber continuously at a suflicient rate of speed to keep down the deleterious substances, and if the proper annealing temperature could be maintained under such circumstances, a desirable result could be secured. However, this is in most instances commercially impracticable, as will be readily understood, and therefore our teachings comprise, as has been indicated, the maintenance of the atmosphere in proper condition. Indeed, within the limitation of ordinary commercial operation, where it is impracticable to provide great volumes of fresh purified nitrogen continuously the maintenance and treatment of the atmosphere as we shall hereinafter describe, is of greater importance than the mere aspect of starting with a purified nitrogenous atmosphere.

We have found that it is particularly important to keep the annealing atmosphere within the furnace low in hydrogen or any source of hydrogen, such as water, during at least the major portion of the anneal.

While we do not wish to limit the description of our new process to any theory, our results indicate that the form of the carbon in-the annealed sheets is generally the most important factor in determining their ductility.

During the high temperature portion of the anneal the carbon present in the common types of electrical steels will be completely in solution.

Dining the cooling portion of the cycle the carbon precipitates and we have made the discovery that the type of gas with which the material is surrounded is a very important factor in determining whether or not this precipitate consists of iron carbide or graphite in the annealed sheet, It is also important that the steel is cooled at a sumciently slow rate through the temperature range in which the carbon precipitates and the graphite is formed (1650 to 1350* F. for 4% Si steel). If the usual commercial type of high temperature anneal for 4% silicon steel is carried on in an atmosphere containing an appreciable percentage of hydrogen we have found that graphitization does not take place to any appreciable extent and that the resulting material is exceedingly brittle. On the other hand.'

we have found that if an essentially neutral or slightly oxidizing atmosphere is maintained throughout the anneal, the carbongraphitizes to such an extent that greatly improved ductility results.

Carbon monoxide and carbon dioxide often build up to fairly high concentrations within the annealing box. We have found that it is advantageous to keep these gases as low as possible throughout the anneal. It is not always possible in commercial size furnaces to keep these two gases to extremely low concentrations, due to the rapidity with which these gases are formed or due to the expense of the large, fresh quantities of inert gas, such as nitrogen, which would have to be added to sweep them out of the furnace- In such cases it is quite advantageous to add oxygen preferably in the form of air to shift the ratio of the concentrations of carbon monoxide and other carbonaceous reducing gases to carbon dioxide until the mixture is as nearly as possible neutral or only very slightly oxidizing with respect to the iron-iron oxide reaction equilibrium.

It is our experience that the very best ductility results are obtained with an absolutely inert annealing atmosphere but since this is impracticable to accomplish on plant size equipment, our teachings are directed chiefly towards achieving our greatly improved ductility by commercial methods.

It is often impracticable, as we have indicated, to maintain a sufficiently dry and pure atmosphere of nitrogen within a commercial furnace by continuously diluting the furnace atmosphere with commercial cylinder nitrogen. We 'have found, however, that it is economical to circulate the annealing atmosphere outside of the furnace through drying means, such as silica gel or activated alumina and return the gas to the box. By this means the furnace atmosphere can be kept to the requisite dryness with only a minimum usage of nitrogen. Any other means of drying the annealing atmosphere would be equally eflective.

The ratios of CO to CO1 which are in equilibrium at any given temperaturewith iron and iron oxide, are known to those skilled in the art and can be found in the literature. In making air additions to keep this ratio at a nearly neutral or slightly oxidizing condition, it will of couise be necessary to limit the air additions to such quantities and rates of addition which will not cause objectionable oxidation of the sheets.

In accordance with what we have pointed out,

we have found that there are several satisfactory sources of nitrogen of which our annealing atmospheres are chiefly composed. We have many times used commercial cylinder nitrogen produced by the liquefaction of air and its subsequent rectification. This source of nitrogen usually is quite low in oxygen and we have found that it is often advisable to add oxygen, preferably in the form of air, in accordance with the above teaching, when it is used. Burned dissociated ammonia can be used and this source of nitrogen may have a certain amount of hydrogen or oxygen present according to the completeness of the combustion when the dissociated ammonia was burned.

If excess oxygen is present in the burned disassociated ammonia it may be unnecessary to add any air during the anneal to keep the atmosphere neutral or nearly neutral. If excess hydrogen is present it may be necessary to add more air than normal to keep the hydrogen from building up to values which will be detrimental to ductility.

Instead of nitrogen, other non-reactive gases, such as argon, helium or others known to be non-reactive with respect to iron, at the temperatures employed in annealing, may be used if the precautions regarding the elimination or control of the impurities which occur within the annealing box, are observed.

We do not wish to be'limited to box annealing since a continuous muille furnace may be used to advantage provided that proper care is taken to prevent the ingress of air and moisture into the purified, inert furnace atmosphere surrounding the sheets.

Wealso do not wish to be limited tosilicon steel sheets alone, since other alloys which are known to have good electrical properties may be treated to advantage according to our invention. For instance, we have treated a great number of electrical steels containing, for instance, arsenic, aluminum, and others, in combination with more than 90% iron, and have found that when treated by the customary annealing methods including those employing protective atmospheres these steels will be brittle, but will have improved ductility. if treated according to our invention.

Thefollowing is an example of the procedure followed in the annealing of 4.5% silicon steel by our new process, which will result in sheets having a hitherto unobtainable combination of lowcore loss and high ductility:

The sheets, after having been rolled, are preferably pickled, washed and dried, provided with a separator consisting of a material such as hydrated lime or other suitable refractory material.. They are then piled into a stack,

, coveredwith an annealing box and charged into cycles necessary for controlling grain size and obtaining the desired electrical properties are well understood in the art and form no part of this invention. We then introduceinto the annealing box nitrogen and commence the drying of the atmosphere within the box by recirculating it through a bed of silica gel .or activated alumina.

If theatmosphere within the annealing box is sufficiently high in oxygen to cause undesirable scaling of the edges of the sheets, the box is flushed with large quantities of nitrogen until the oxygen has been lowered to an ineffective value.

. During the heating portion of the annealing cycle large quantities of water vapor are given oft from the charge and its coating. A high rate of recirculation is maintained at this phase of the anneal to get the furnace atmosphere dry as early as possible.

The concentration of hydrogen within the box often reaches a high value at this point in the cycle, largely due to the decomposition of water vapor by the steel sheets. With the proper amount of drying the moisture content of the annealing atmosphere willbe reduced to such a concentration that its dew point will be at least as low as 10 C. by the time the maximum annealing temperature is attained. A dew point as low as 20 C. has often been attained during the high temperature soaking period of commercial anneals.

During the middle or latter portion of the heating-up period, the maximum concentrations of CO and C02 will be experienced, after which there will be a gradual diminution. Following these compositions by well known means of gas analysis, the operator adds air to the furnace or gas recirculation system in suflicient quantities to keep the CO/COz ratio nearly neutral or slightly oxidizing. Later during the anneal the air addition rate can often be decreased or stopped as the reducing gases fall off to negligible concentrations.

During the high temperature portion of the cycle it may be unnecessary to recirculate and dry the annealing atmosphere at the high rate which was used at the first portion of the cycle to maintain a-low dew point. It may again be desirable to increase the recirculation rate during the cooling portion of the cycle to cool the charge more rapidly and thereby shorten the total annealing time. This must be done with care, however, to avoid warping the sheet by too rapid cooling. u

These secondary manipulations can be carried out in any manner desired providing the teachings with regard to the maintenance of the proper atmosphere vithin the annealing box are obing operation. During the first part ofthe an nealing process another protective gas may be passed through the annealing box such as hydrogen. Due to the reducing effect of such gases beneficial results may be obtained such as the removal of sulphur and other impurities from the sheets. However, these gases should not be allowed to remain for too long a time in contact with the sheets as brittleness may result. Atany rate, our improved, purified inert gas atmosphere must be applied not later than during the soaking period of the annealing cycle and be maintained until the sheetshavecooled down to a safe temperature at which no scaling of the sheets will occur.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A method of heat treating silicon steel sheets which comprises annealing the sheets in a confined atmosphere of a non-reactive gas substantially free from moisture, hydrogen and oxygen, in any form, continuously passing the used inert gas through purification means thereby substantially removing any moisture, hydrogen and oxygen which may have been taken up in any form during the contact with'the sheets and recirculating the purified inert gas.

2. A method of heat treating silicon steel sheets to produce sheets having more than the normal ductility of heat treated silicon steel sheets, which comprises annealing said sheets in-a space in which the atmosphere can be controlled, introducing into said space an atmosphere principally of nitrogen, recirculating the annealing gas and during said recirculation, treating the recirculated gases so that at least as early as the soaking period of the annealing cycle, the sheets are surrounded by a nitrogenous atmosphere kept continuously free from hydrogen and oxygen and combinations thereof to the extent that any residual quantities of them will have no appreciable embrittling effect upon the sheets.

3. A method of heat treating silicon steel sheets which comprises annealing the sheets in an atmosphere of nitrogen from which moisture, hydrogen and oxygen have been substantially removed, re-circulating said atmosphere and treating it for the removal of moisture and maintaining said atmosphere in a nearly neutral condition with respect to iron and iron oxide by adding oxygen to the point of maintaining a substantially non-reactive balance of CO and C02.

4. A method of heat treating silicon steel sheets which comprises annealing the sheets in an atmosphere the principal component of which is nitrogen, and continuously removing from said of oxidizing reactive gases containing oxygen and oxidising hydrogen by controlled admission of air to the gases.

5. A method of heat treating silicon steel sheets which comprises annealing the sheets in an atmosphere the principal component of which is nitrogen, and continuously removing from said atmosphere moisture and hydrogen and omen in reactive .iorms, the latter step including the step of oxidizing reactive gases containing oxygen and oxidizing hydrogen by'oontrolled admission oi air to the gases, said annealing being carried on at a temperature above 1350' It, and terminating said annealing treatment with a slow cooling within the temperature rangeof 1650' to 1350' l". g

6. A method of heat treating silicon steel which comprises annealing in an atmosphere, the principal component of which is nitrogen, continuously removing moisture from said atmosphere and bringing the concentration of the oxidizable gases present substantially into equilibrium with their oxidation products with respect to iron and iron oxide by the controlled admission of air to the gases. e

'l. A method of heat treating silicon steel sheets which comprises annealing the sheets in a coniined atmosphere oi nitrogen and removing impurities from the nitrogen in contact with the sheets during the anneal so that at least during the high temperature soaking and cooling portion of the annealing cycle, the nitrogen will be free from moisture, hydrogen and oxygen including impurities which may have been taken up by the nitrogen during the contact with said sheets, to such an extent that any residual quantities of said substances in contact with the sheets will purities from the nitrogen in contact with the 15 sheets during the anneal so that at least during the high temperature soaking andcooling portion of the annealing cycle,- thenitrogen will be free from moisture, hydrogen and oxygen including impurities which may have been taken up by the nitrogen during contact with said sheets, to such an extent that any residual quantities oi" said substances in contact with the sheets will have no appreciable embrittling eflect uponthe said sheets. the said removal. being eiiected, in 25 part at least, by passing through the furnace such quantity of nitrogen as to remove substantial portions of said substances by dilution.

ROBERT L. DAVIDSON. CARL C. MAHLIE.