US3201293A - Coated silicon iron sheet stock - Google Patents

Description

1965 v. w. CARPENTER ETAL 3,201,293

JOHN M JACKSON AND BY ROBERT W. Saunas, MMMM, 96mm Haifa arronuzvs.

United States Patent 3,201,293 COATED SILMON IRON SHEET STOCK Victor W. Carpenter and John M. Jackson, Middletown, and Robert W. Squibb, Zanesville, Qhio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Original application Nov. 12, 1%8, Ser. No. 773,419.

Divided and this application Feb. 27, 1963, Ser. No. 261,270

1 Claim. (Cl. 148-3155) This is a division of the copending application Serial No. 773,419, now abandoned, filed November 12, 1958, in the names of the same inventors and bearing the same title.

The invention pertains to magnetic materials which can be made by strip 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 silicon iron, without limitation.

By non-oriented silicon iron is meant a product devoid of such a degree of preferred orientation of the crystals as would produce marked differences in magnetic permeability of the product as measured in the rolling direction and in a direction at right angles thereto. Nonoriented silicon irons are of utility in the manufacture not only of rotating electrical machinery, but also in the manufacture of transformers the cores of which are made up of E-shaped, I-shaped, or L-shaped laminations. In such uses rotating elements such as motor armature laminae and transformer laminations are formed from the steel sheet 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 nonaging and at the same time characterized by a high die life.

This primary object of the invention and others, 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. Reference is made to the accompanying drawings wherein:

FIG. 1 is a photornicrograph at 1000 magnification of a piece of silicon iron sheet stock which has been processed in a typical fashion as hitherto known in the art.

FIG. 2 is a photomicrograph at l000 magnification of silicon iron stock which has been processed in accord ance with this invention, and showing a novel surface condition as compared with the product of FIG. 1.

The figures are micro sections made in the standard way, the lower layer being the silicon iron, and the upper layer being a layer of copper or the like. Between the two layers is an indication of the surface condition on the silicon iron produced by the process steps hereinafter discussed.

In the manufacture of non-oriented silicon iron sheet stock in the past, it has been a common practice to produce thin bar by hot rolling and then subject the thin bar to a decarburising treating. 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 ice material to final gauge and the welding step, all preliminary to a final anneal at a relatively high temperature, namely, a temperature of about 2050" F. It is generally understood that, in order to produce the opti mum magnetic characteristics which the product is capable of acquiring, 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 suffers 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 process of the present invention, 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 form which will require welding to form it into a coil of indefinite length. But the principles of this invention 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 the practice of the invention, 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 as 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 for the material in accordance with the present invention 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 temper roll it, pickle it, and then again 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 is not subiected 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 decarburizing atmosphere. Next, the material is treated to a continuous high temperature anneal at about 2050" F. in a dry, non-decarburizing atmosphere. It will be 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 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 decarburizing agents. When hydrogen or hydrogennitrogen atmospheres are used, a dew point about 125 F.i25 is maintained by adding water. When cracked gases are used, the dew point is somewhat lower, since CO is available as an additional decarburizing agent. The annealing treatment can normally be accomplished in 1 to 3 minutes at a temperature substantially within a range of 1350 to 1650 F.

By a dry non-decarburizing atmosphere is meant atmosphere that is substantially non-oxidizing to carbon but is also not carburizing. Such an atmosphere can be hydrogen, nitrogen, or mixtures thereof having a dew point less than +50 F. The temperature for the final continuous anneal should be within the range of substantially 1800 to 2200 F. Higher temperatures may be employed if desired but are generally found uneconomical.

if the decarburizing anneal, which has been described above, were the last step in the routing, it will be evident to one skilled in the art that the proper grain size and optimum magnetic properties in the material would not be developed. A non-aging product could be made by conducting a final high temperature anneal in a decarburizing atmosphere; but it has been found that when this is done, the die life is very seriously affected, bumps form very rapidly on the hearth rolls, and the surface skin of the silicon iron strip can be loosened by bending or punching. When a silicon iron sheet stock is formed by the typical prior art process outlined above, i.e. by a procedure involving decarburizing at a thin bar gauge followed by further reduction and a high temperature anneal, a material is produced which not only may be subject to aging, but is characterized by relatively poor die life. The surface condition of such a product is illustrated in FIG. 1, which is a photomicrograph of a material rolled from decarburized bars, pickled, welded, and strip annealed at 2050 F. in dissociated ammonia. The dark line in the photomicrograph shows a layer of silica particles coalesced by the high temperature anneal. These silica particles adversely affect die life; and the material of FIG. 1 gave a die life of only about 20,000 strokes. The die life would not have been improved had the final anneal been carried on in a decarburizing atmosphere, although under these circumstances, a diminution of the aging tendency might have been achieved.

It has been found that a heat treatment in a decarburizing atmosphere will give a reasonably satisfactory die life on previously pickled material only up to about 1700 E. which is not high enough to develop the optimum magnetic properties of the material. A decarburizing treatment oxidizes silicon, producing at and near the surfaces of the stock a distinct layer comprising a very substantial quantity of silica. in the manufacture of highly oriented silicon iron stocks, it has sometimes been the practice to decarburize just prior to a final high temperature box anneal. Under these circumstances, an annealing separator must be used; and the result is the finished product is characterized by a glassy coating which, while it may under some circumstances have utility as an interlamination insulator, is destructive of die life.

When, however, the particular series of process steps outlined above as characteristic of this invention is followed, a product is produced which is not only non-agin g but has remarkable die life characteristics. The surface condition of this product is illustrated in H6. 2 which shows, as 1000 magnification, the surface of a silicon iron which has been hot rolled to gauge, pickled, welded, treated at 1475 F. in a decarburizing atmosphere, and then continuously annealed at 2050 F. in a dry non-decarburizing atmosphere. Surprisingly, the surface of such material exhibits a very thin band of dark oxide at about the mid-thickness of the surface skin. Without prejudice, the theory is that this band or line 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 took place below this band, raising the band to about the mid-thickness of the final surface skin. Had a continuous thin band 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 grayish material in the photomicrograph, which grayish material is believed to be iron containing various refractory oxides. Although the final skin has an appearance indicative of the presence of particles of silica, these are in some way cushioned to the extent that the product has a remarkable die life. Moreover, the final skin is non-peeling.

The appearance of the sheet surface of silicon iron stock, made by the prior art procedure and having the characteristics illustrated in FIG. 1, is dark, while the stock of this invention has a lighter silvery visual appearance. The product of this invention will blue very readily when heated in an open flame, indicating an iron surface, while the prior art material blues much less readily. The surface skin on the material of this invention is not loosened by repeated bending of the stock.

The practice of the present invention requires that the decarburizing anneal be kept separate from the final high temperature anneal. in separate furnaces where the atmospheres can be carefully and independently controlled. It is possible for economic reasons to carry on both operations in a composite furnace structure; but where this is done, the furnace should consist of a first section for decarburizing with a ccuntercurrent flow of the dccarburizing gas, a middle transition section wherein the stock can be maintained under a neutral atmosphere and which serves to isolate the decarburizing section from the high temperature section of the furnace, and a final high temperature section characterized by concurrent fiow of a dry nondecarburizing atmosphere, as herein defined. Precautions should be taken against mixing of the atmospheres in the composite furnace except in the transition section, from which the mixed gases can be exhausted.

Example A silicon iron stock containing 3.0% silicon was hot rolled to final gauge in sheet form. It was then temper rolled, pickled, and again temper rolled,'after which the sheets were welded together so as to form a coil for continuous annealing. The coil was treated at 1475 F. in a dccarburizing atmosphere, as herein defined, for about three minutes at temperature. It was then subjected to a strip anneal at 2050 F. in a dry non-decarburizing atmosphere as above defined, the atmosphere also consisting of dissociated ammonia. The duration of the high temperature heat treatment was about two minutes.

The material so treated was non-aging and had a die life of 135,000 strokes as compared with a die life of 20,000 strokes for a material of the same composition processed in accordance with the prior practice set forth herein.

While the process has been described in connection with c routings particularly appropriate to the manufacture of non-oriented material, it will be understood that the routing steps prior to the final strip decarburizing and strip annealing procedures may be varied. The skilled worker in the art will understand that the production of oriented grades of silicon-iron generally involve a plurality of carefully controlled cold rolling reductions with intermediate anneals also of carefully controlled nature, and a final high temperature anneal which may be the strip anneal of this process. Still other procedures involve. intermediate high temperature treatmentst These operations can be carried on Modifications may be made in the invention Without departing from the spirit of it. The invention having been described in an exemplary embodiment, what is claimed as new and desired to be secured by Letters Patent is:

A silicon iron sheet stock containing substantially .5 to 3.8% silicon, having a carbon content not substantially in excess of 0.01%, being non-aging and characterized by an excellent die life, the said stock having a surface skin of light gray color derived during heat treatments from the silicon iron sheet stock itself, and characterized, under microscopic examination, by a layer of silica at about the mid-section of the surface skin and separated from the surface of the stock itself, as well as covered, by a softer material consisting in part at least of iron.

References Cited by the Examiner UNITED STATES PATENTS 10 DAVID L. RECK, Primary Examiner.

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