US3196054A - Process of decarburizing and annealing of open coil silicon-iron sheet stock without intervening surface treatment - Google Patents

Description

United States Patent "'ice P38532553 0F DEQARBUREZHNG AND ANNEALWG (3F @PEN 8312 SlLlCON-ERQN SHEET STOCK WlTl-IQUT HNTERVENING SURFACE TREAT- MENT Victor W. fiarpenter and John l t l. Jachson, lviiddletovvn, and Robert W. anihh, Zanesviile, Ghio, assignors to Armeo Steel Corporation, Middletown, (thin, a corporation of ilihio No Drawing. Filed Aug. 14, E63, Ser. No. 391,959

8 (Ziaims. (til. 148-111) The invention pertains to magnetic materials which can be made by relatively inexpensive routings including simple and inexpensive annealing methods. Much of this type of product is of a non-oriented nature, but aspects of this invention can also be used in the manufacture of oriented products where the particular qualities imparted by the essential sequence of steps taught herein are of importance. The invention will, however, be described in connection with the production of non-oriented siliconiron without limitation.

By nonrientcd silicon-iron is meant a product devoid of such a degree of preteired orientation of the crystals as would produce marked differences in magnetic permeability of the product as measured in the rolling direction and in other directions at various angles thereto. Non-oriented silicon-irons are of utility in the manufacture not only of rotatin electrical machinery, but also in the manufacture of transformers the cores of which are made up of E-shaped, I-shaped, or other laminations. In such uses rotating elements such as motor armature laminae and transformer laminations are stamped, punched or sheared from the steel sheet or coil stock by means of dies; and the importance of a good die life is obvious.

A primary object of the invention is the provision of a silicon-iron stock of either type which will be non-aging and at the same time characterized by a high die life.

It is an object of the invention to provide a procedure in which use can be made of open coil annealing steps as later Set forth.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished by that procedure of which an exemplary embodiment will now be described.

In the manufacture of non-oriented silicon-iron sheet stock in the past, it was a common practice to produce thin bar by hot rolling and then subject the thin bar to a decarburizing treatment. Thin bar is usually a material about .1 in. in gauge, although the exact thickness is not critical. The thin bar, after decarburizing, was reduced to final gauge, usually by hot rolling; and if originally produced in the form of sheets, the sheets were welded end to end to form a coil suitable for continuous annealing. Various temper rolling and pickling steps might be employed intermediate the rolling of the material to final gauge and the welding step, all preliminary to a final anneal at a relatively high temperature. It is generally understood that, in order to produce the optimum magnetic characteristics which the product is capable of acquiring, and including the optimum core loss values, a high temperature final anneal is necessary.

Products made as above indicated have many desirable characteristics; but they may have erratic core loss behavior generally known as aging, in which the product in instances suifers a change in core loss with the passage of time, a phenomenon believed to be due to an instability which gives rise to a tendency to precipitate carbon compounds.

In the copending application of the same inventors, Serial No. 262,288, filed February 27, 1963, which is a fiflihhfiliid Patented July 20, 1965 continuation-in-part of application Serial No. 773,419, filed November 12, 1958, and bearing the same title, a procedure is taught which will now be outlined. The material is reduced to gauge in any suitable way. The reduction may be accomplished either by hot rolling or by cold rolling, or by combinations of the two, which is a distinct advantage in processing. The product may be hot rolled to thin bar, and then further hot rolled to gauge, giving a product in sheet from which will require welding to form it into a coil of indefinite length. The essentials of the process are equally applicable to a material which is hot rolled from slabs into long coils, and thereafter pickled and cold rolled to final gauge while still in coil form. The final thickness of the material may be substantially in the range of 24 to 29 gauge.

Silicon-iron suitable for use in the process of the copending application and in the process hereinafter outlined, particularly in the manufacture of non-oriented stock, may be defined as a material containing substantially 0.5% to about 3.8% silicon. Aluminum may be present in quantities up to about 0.5%. The carbon content initially is not a limitation on the invention but usually varies from about 0.02% to about 0.08% in accordance with the manner in which the steel has been made. In addition to silicon, aluminum and carbon, the alloy may contain such amounts of other elements, e.g. manganese, phosphorus, sulfur and the like as are usual in silicon-irons produced commercially. The balance of the alloy will be substantially all iron. The analysis given above relates to the material after processing has started, i.e. it is not a ladle analysis; but it may be the analysis of the silicon-iron immediately before or immediately after the hot rolling.

An exemplary routing may be given as follows:

The material is hot rolled, or hot rolled and cold rolled, to final gauge as stated above.

If the material has been hot rolled to final gauge, it will be usual to pickle it and then temper roll it prior to welding the individual sheets end to end to form a coil. If the material has been cold rolled to final gauge in sheet form, the temper rolling and pickling steps may be omitted, but the individual sheets will then be welded end to end to form a coil. If the material has been produced in coil form by cold rolling to gauge, the welding step will, of course, be unnecessary. It will be noted that the material need not necessarily be subjected to a decarburizing treatment at an intermediate gauge.

The material, produced as indicated, is first subjected to a continuous decarburizing anneal at about 1475" F. in a decarburlzing atmosphere in accordance with the teachings of the copending application. Next, the material is treated to a continuous high temperature anneal in a dry non-decarburizing atmosphere. It was found not only that the material is devoid of aging characteristics, but also that it has an excellent die life, as will be hereinafter more fully set forth.

By a decarburizing atmosphere is meant an atmosphere of Wet hydrogen, hydrogen-nitrogen mixtures containing water vapor as the decarburizing agent, or cracked gases such as DX, which, in addition to nitrogen, hydrogen and carbon monoxide contain carbon dioxide, and Water vapor as the decarburizing agent. When hydrogen or hydrogen-nitrogen atmospheres are used, a suitable dew point is maintained by adding water, usually as steam. When cracked gases are used, the dew point is somewhat lower, since CO is available as an additional decarburizing agent. The continuous decarburizing treatment of the copending application can normally be accomplished in one to three minutes at a temperature substantially within a range of 1350 to 1650 F., in an atmosphere of dissociated ammonia having a dew point about F.i25

By a dry non-decarburizing atmosphere" is meant an atmosphere that is substantially non-oxidizing to carbon but is also not carburizing. Such an atmosphere can, in the copending application, be hydrogen, nitrogen or mixtures thereof having a dew point less than +50 F. The temperature for the final continuous anneal in the dry atmosphere was stated to be within the range of substantially 1600" to 2200 F. Higher temperatures may be employed if desired but are generally found uneconomi cal. Permissible variations in temperature will be discussed more at length hereinafter.

Were a silicon-iron sheet stock to be formed by the typical prior art process first outlined above, i.e. by a procedure involving decarburizing at a thin bar gauge followed by further reduction in gauge and a high tem perature anneal, a material would be produced which not only is subject to aging, but is characterized by relatively poor die life. If such a product were sectioned and examined microscopically, the surface or skin portion would exhibit a dark line believed to be a layer of silica particles ,coalesced at the surface of the silicon-iron stock by the high temperature anneal. These silica particles would adversely affect die life.

When, however, the particular series of process steps outlined above as characteristic of the copending application is followed, a product is produced'which is not only non-aging but has a remarkable die life. In general, if such a product is sectioned and examined microscopically,

it shows a very thin dark band of oxide at about the mid- ,thickness of the surface skin. The surface skin, more- .over, is non-peeling. The results obtained by the process of the copending application are unique and highly valuable. It has been believed, however, that the process could be carried on only by utilizing annealing treatments of the nature of strand or continuous anneals. These have the advantage of free access of the furnace atmosphere to the surfaces of material being treated; but they have certain disadvantages as well. Continuous or strand anneals are most efficient when practiced upon lighter gauge materials, say less than about 24 gauge, and preferably substantially thinner. If continuous annealing techniques are to be applied to materials of 24 gauge or heavier, it will take longer to bring such materials up to temperature. This might make for larger and more expensive furnaces, or the use of higher furnace temperatures or both. Moreover, the continuous furnace is more wasteful of special atmospheres which may be expensive, and also presents a more difiicult maintenance problem.

Before describing the variants of process steps contemplated by the present invention, certain general considerations should be noted.

To attain a magnetic material having the qualities produced in the copending application, it is necessary that a wet gas decarburization step be practiced. Unless carbon is reduced to a low level, the material will not have the best magnetic characteristics, and it will also be subject to aging. To develop the best magnetic qualities which can be obtained from a material of given chemistry, a high temperautre anneal is necessary. If the final anneal is a heat treatment at about 1900 F. or above, in a .dry (non-decarburizing) atmosphere, some sort of glaze forms on the surface of the material which seriously interferes with die life. If moisture is introduced into the atmosphere of a final high temperature'anneal, the glaze will not form; but there will be a considerable oxidation of silicon to silica just beneath the surface skin on the product because a wet atmosphere is oxidizing to carbon and silicon although it may be neutral or reducing toward iron. This also seriously interferes with die life.

In the process of the copending application it was found that if the sheet stock is first subjected to a wet gas decarburizing treatment at a relatively lower temperature, and then to a dry anneal at a higher temperature, the peculiar effects noted in the copending application are obtained. No great amount of silica appears to form during the high temperature anneal and there willbe no glaze. Also the silica and other oxides appear, as noted above, to form a layer at about the mid-thickness of the surface skin. Without prejudice, the theory is that this band or layer represents the original location of the interface between the base metal and the skin formed during the decarburizing operation. In the final high temperature anneal in a dry non-decarburizing atmosphere some slight oxidation of silicon and aluminum may take place below this band, raising the'band to about the mid-thickness of the final surface skin. Had a continuous thin band or layer of silica been formed at the interface with the base metal during the final high temperature anneal, it is likely that the surface skin would have been subject to peeling, but as indicated, a slight oxidation has occurred below the original interface layer, the net result being that the original silica skin and additional particles .of silica and alumina are embedded in a mastic of greyish material, which greyish material is believed to be iron containing various refractory oxides. Although the final skin has an appearance indicative of the presence of par ticles of silica, these are in some Way cushioned to the extent that the product has an excellent die life. Moreover, the final skin is non-peeling.

So far as in known, the ordinary techniques of boX annealing are not available for carrying on any part of the two consecutive heat treatments taught into the copending application. If one attempted to use stacked sheets or tightly wound coils of metal, it would be found necessary to employ an annealing separator. Any residue of annealing separator upon the surfaces of the finished stock, would be detrimental to die life. Consequently a cleaning operation would be required. Again, it will be clear that the presence of an annealing separator between the layers of stacked sheets or the convolutions of a coil would interfere with access to the gases of the furnace atmosphere to the sheet surfaces. Yet again, there are certain annealing separators, e.g. magnesia, which tend to combine with the silica in a high temperature heat treatment to form a thin coating of glass. Such a coating, while for some products it might have utility as a means of obtaining interlamination resistivity, nevertheless is highly detrimental to die life and is not desired on products which are to be fitted for ultimate use by stamping in dies.

Various procedures have been developed for annealing metal in mufile-type furnaces, where the metal exists in the form of loose coils with sufficient space between the convolutions to permit the furnace gases to get at all of the surfaces of the stock. Loose coils are sometimes formed by winding the strip metal onto a mandrel with the interposition of a strand-like material such as nylon. When the coil has been completed, the strand material may be pulled out from between the convolutions leaving the coil in a loose condition. This procedure has a certain disadvantage in that after removal of the strand material the convolutions may touch each other at various points. A preferable procedure involves the use of a heat resistant strand-like element which may be wound between the convolutions when forming a coil, but which may be left in place during the annealing, the strand-like material being characterized by protuberances of such depth and so placed that the furnace atmosphere can readily enter between the convolutions despite the continued presence of the strand-like material.

In furnaces equipped for loose coil annealing, the coil will generally be supported on end on a base having perforations, grooves or the like therein, so that the annealing atmosphere may pass into the spaces between the coil convolutions on one side of the coil and exit from between the convolutions on the other side. Some furnaces designed for loose coil annealing are provided with means for circulating the furnace atmosphere and forcing it to enter and flow between the convolutions of the coil or coils within the mufiie. Some elaborate furnace structures have also been devised in which the coils rest on oneness a turn table, and the subjected to different conditions within difierent parts of the muffle itself.

Loose coil annealing has certain advantages in that it is more economical or" gas and can be more accurately controlled, in general, as to atmosphere and temperature. It has certain disadvantages in that the times involved for the treatment of any given area of the material being processed are very much longer and the furnaces are not ordinarily designed for heat treatments at as high a temperature as the highest temperatures which may be obtained in continuous furnaces. At the same time, it will be evident that overheating of exterior surfaces of magnetic sheet stock may readily be avoided because of the longer times involved.

From the discussion above, it will be seen that open coil annealing would not be considered by the skilled worker in the art as suitable for the practice of the annealing steps set forth in the copending application. Certain of the reasons for this may be summarized as follows:

(a) There will be more oxidation during decarbu-rization because of the longer time involved,

(b) There will be more nitriding, if nitrogen is available in the decarburizing atmosphere, as in the use of dissociated ammonia,

(c) There will be a greater tendency for the surfac to fiake,

(d) The grain size will be larger because of the increase in time at temperature, and

(c) It might be difficult if not impossible to develop the ultimate magnetic characteristics by a high enough temperature in the material.

It has, however, been found that the-re are ways of circumventing these disadvantages.

It may be restated that we are dealing here with a Wet gas decarburizing treatment at preferably a relatively lower temperature followed by a dry gas heat treatment at preferably a relatively higher temperature.

The latter heat treatment will hereinafter for brevity be called the final anneal, it being kept in mind, of course, that after the sheet stock has been punched into laminations, the laminations may be given a low temperatur strain-relieving anneal by the customer.

Open coil annealing techniques can be applied either to the decarburizing step or to the final anneal or to both. The stock may be decarburized in a muffle in a wet decarburizing atmosphere as above defined, thereafter being cooled in the mufiie and then subjected to an open anneal in a dry non-decarburizing atmosphere. The disadvantages here are the possibility of greater oxidation and nitriding in the decarburization step as well as some increase in grain size. But it has been found that thes difficulties can be minimized by carrying on the wet gas decarburization at a lower temperature in an atmosphere characterized by a lower dew point. A top temperature of about 1300 F. is preferred; but good results can be obtained at temperatures extending from about 1200" of about 1500 F. Instead of a dew point of about l25i25 F., a lower dew point of 9fii20 F. should be used. While these ranges overlap those of the cope-riding application to some extent, the accent is on the lower temperatures and the lower dew points, and the use of both is rendered possible by the greater length of time the stock remains in heated condition in the furnace.

The use of open coil techniques for the final anneal involves somewhat similar difiiculties. But whereas in the copending application the desired results for the final anneal were attained in a continuous furnace containing a non-decarburizing atmosphere at a dew point of less than +50 F. by heating the material preferably to a temperature of 1-600" to 2200 F. for a length of time of about a minute or less, it has now been ascertained that a satisfactory product will be produced at a temperature of around 1500 to 1600 F. with a soaking time of about one-half to three hours. The general range of temperature may be given as about 1400" to 1900 F. Here the extended time of the heat treatment makes possible the development of ultimate magnetic characteristics at lower temperatures, and excessive grain growth is minimized at such lower temperatures.

To the extent that a final open coil anneal would tend to produce a product requiring flattening when decoiled, an extra flattening anneal at low temperature (about 1300 to 1500 F.) may be practiced as required. This is a continuous heat treatment under conditions of tension suillcient to flatten the stock Without subjecting the stock to appreciable working. Flattening anneals may be practiced in various ways. For an exemplification of one procedure satisfactory for the purpose reference is made to the copending application of Lowell L. Cook, entitled Thermal Flattening of Metallic Strip, Serial No. 782,214, filed December 31, 1958.

In processes in which both the Wet gas decarburization and the dry gas final anneal are practiced on an open coil in a single mufile furnace, certain advantages are obtained. The use of continuous furnaces is obviated (excepting as to a flattening anneal where necessary) with a consequent savings in equipment cost and capital outlay. Savings are also made in the cost of special atmospheres for annealing. Moreover, the process can be practiced upon heavy gauge as well as light gauge stock without a change in equipment.

Example I A silicon-iron is cast into ingot form and is then hot rolled to an intermediate gauge in a continuous hot strip mill. The hot rolled material is then shot blasted and/ or pickled to remove the hot mill scale and is then cold rolled to a desired gauge which may be as heavy at 24 gauge or heavier. The cold rolled stock is formed into an open coil in any of the ways set forth above. The open coil is placed in an annealing mutlle with provision for atmosphere control and preferably provision for the circulation of atmosphere Within the muffle so that all portions of the strip surfaces will be contacted by the atmosphere.

The mufile is heated so as to raise the temperature of the metal in the open coil to, say, about 1350 F. The atmosphere in the mufiie, at least during the soaking period of the decarburizing step, may be an atmosphere of hydrogen or dissociated ammonia so treated by the introduction of water vapor as to have a dew point of approximately 'i-20 F. The decarburizing treatment in such an atmosphere may be continued at temperature until the carbon in the stock has been reduced to the desired total value, say 0.005% or less.

Thereafter the atmosphere within the mufiie will be changed so that it consists of hydrogen or dissociated ammonia having a dew point no greater than about 50 F. This can be done by introducing the dry atmosphere into the mufiie in sufficient quantity to sweep the wet atmosphere out of it, and it is preferably done prior to any increase in the temperature of the product for the second anneal, say an increase in temperature to about 1600 F.

Under the conditions outlined, effective decarburization can be carried on at temperatures substantially Within the, range of 1200 to 1500" F. with soaking times of approxi: mately one to eight hours, the time preferably varying inversely with the temperature.

The final annealing step can be accomplished at temperatures ranging from about l400 to 1900 F. with minimum soaking times of about one-half to three hours. Again the times preferably vary inversely with the temperatures. Depending upon the characteristics of the mufile and the manner in which it is heated, there may be an upper feasible limit of temperature which can be attained therein.

After the final annealing step the stock is cooled in the muffle. Subsequently, if required by the condition of the stock, it may be subjected to a flattening anneal as above described.

In connection with the above example, it may be pointed out that, although the ranges of temperature given for the decarburizing and final annealing steps overlap, it is preferable that the final anneal be carried on at a temperature higher than that of the decarburizing step. The times given are in the nature of times which will generally be found satisfactory; but the time at temperature in the decarburization step may be varied and either increased or decreased depending upon the initial carbon content of the stock and its desired final carbon content. In the dry final anneal the ultimate magnetic properties of the strip will be developed. While different permeabilities may be desired or tolerated in different magnetic stocks, depending upon the usage to which they will be put, the temperature and soaking time are again interrelated in that a given degree of magnetic improvement may be attained with a longer soak at a lower temperature within the range given and vice versa. To increase the permeability of the stock the times set forth may be lengthened assuming that such increased times do not produce a greater than desired grain size.

The product of the process is like that of the copending application in the names of the same inventors, in that the product has a light silvery visual appearance. Also the product blues very readily when heated in an open flame, indicating an iron surface. The surface skin of the material is not loosened by repeated bending of the stock.

At the same time the dark line seen in photomicrographs of the product of the copending application at about the mid-section of the surface skin is not as pronounced or may be visually undetectable. Nevertheless advantages are obtained in the finished stock equivalent to the advantages inherent in the product of the copending application. If the product of this invention is decarburized as set forth in the example, it is believed that the slower and more gradual decarburization results in the formation of silica or other refractory oxi es in finer particles distributed in the surface skin, as distinguished from a tightly adherent layer of coalesced silica or other refractory oxides on the surface of the metal. However, merely decarburizing the product in open coil form in a mufile as herein taught does not give a product having optimum magnetic properties. The final anneal in a dry atmosphere is necessary not only for the improvement of magnetic qualities but also to insure excellent die life.

If the decarburizing anneal and the final anneal were to be carried on in a wet decarburizing atmosphere the die life would be quite poor. This is believed to be due to the excessive oxidizing etfect of a wet atmosphere at the final anneal temperature. As a consequence the atmosphere in themufile should be changed from a wet to a dry atmosphere as defined as soon as possible after the attainment of the desired degree of decarburization. While the overlapping of the temperatures given above for the decarburizing and final annealing treatments might suggest that both treatments could be carried on at the same temperature, this is not generally conducive to the optimum results. Moreover, a gradual change within the mufi'le from a wet decarburizing atmosphere as defined to a dry final annealing temperature as defined, is also productive of relatively poor results. This may be due to the fact that the persistence in the atmosphere of such a quantity of moisture as will render it oxidizing to silicon, aluminum or in some instances iron, will tend to produce too much oxide at the higher temperature. It furthermore appears necessary that the final heat treatment be carried on in an atmosphere which will act to reduce a very substantial quantity if not all of any oxide of iron formed in the treatment.

As a consequence the atmospheres used in the two stages of the treatment should be kept distinct, and the change over from one atmosphere to the other should be accomplished as rapidly and thoroughly as possible.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in certain exemplary embodiments, what is claimed as new and esired to be secured by Letters Patent is:

1. In a process of producing non-aging silicon-iron sheet stock having a good die life, in which the said silicon-iron is reduced to a desired final gauge and is pickled subsequent to any hot rolling practiced in the reduction, the combination of steps consisting of subjecting the reduced stock to a decarburizing anneal in a wet gas decarburizing atmosphere at a temperature in the range of 1200 to 1500" F, and then without intervening surface treatment subjecting the reduced stock to an anneal in a dry non-decarburizing atmosphere at a temperature of substantially 1490 to substantially l9GO F. for developing the ultimate magnetic charactersitics of the sheet stock, at least one of the said anneals being carried on in a mufile with the sheet stock in the form of an open coil.

2. The process claimed in claim 1 wherein the decarburizing anneal is carried on in a muffle with the stock in the form of an open coil in a hydrogen bearing atmosphere having a dew point of about i-20 F. with a time at temperature of about one to about eight hours.

3. The process claimed in claim 1 wherein the said anneal for the development of magnetic characteristics is carried on in a muffle with a material in the form of an open coil, in a hydrogen bearing atmosphere having a dew point not substantially higher than about 50 F., with a soaking time of about one-half to three hours.

4. The process claimed in claim 1 in which both annealing steps are carried on in a single mufile with the stock in the form of an open coil, the said decarburizing anneal being performed in a hydrogen bearing atmosphere having a dew point of substantially 90i20 F. with a soaking time at temperature of about one to about eight hours, and wherein the final anneal is carried on in a hydrogen bearing atmosphere with a dew point not greater than about 50 F. and with a time at temperature of about onehalf to about three hours.

5. The process claimed in claim 4 in which the temperature of the said final anneal is substantially higher than the temperature of the said decarburizing anneal.

6. The process claimed in claim 4 in which the temperature of the said final anneal is substantially higher than the temperature of the said decarburizing anneal, and in which the atmosphere in the mufile is changed from the atmosphere having the higher dew point to the atmosphere having the lower dew point prior to the attainment of the said higher temperature.

7. The process claimed in claim 6 in which the product subsequent to the said final anneal for the development of ultimate magnetic properties is subjected to a low temperature flattening anneal.

8. The process claimed in claim 7 wherein the stock has a thickness not less than about 24 gauge.

References Cited by the Examiner UNITED STATES PATENTS 2,287,467 6/42 Carpenter et al l48110 3,089,795 5/63 Hu 148111 FOREIGN PATENTS 870,220 6/61 Great Britain.

DAVID L. RECK, Primary Examiner.

Claims (1)

1. IN A PROCESS OF PRODUCING NON-AGING SILICON-IRON SHEET STOCK HAVING A GOOD DIE LIFE, IN WHICH THE SAID SILICON-IRON IS REDUCED TO A DESIRED FINAL GAUGE AND IS PICKLED SUBSEQUENT TO ANY HOT ROLLING PRACTICED IN THE REDUCTION, THE COMBINATION OF STEPS CONSISTING OF SUBJECTING THE REDUCED STOCK TO A DECARBURIZING ANNEAL IN A WET GAS DECARBURIZING ATMOSPHERE AT A TEMPERATURE IN THE RANGE OF 1200* TO 1500*F., AND THEN WITHOUT INTERVENING SURFACE TREATMENT SUBJECTING THE REDUCED STOCK TO AN ANNEAL IN A DRY NON-DECARBURIZING ATMOSPHERE AT A TEMPERATURE OF SUBSTANTIALLY 1400* TO SUBSTANTIALLY 1900*F. FOR DEVELOPING THE ULTIMATE MAGNETIC CHARACTERISITICS OF THE SHEET STOCK, AT LEAST ONE OF THE SAID ANNEALS BEING CARRIED ON IN A MUFFLE WITH THE SHEET STOCK IN THE FORM OF AN OPEN COIL.