US3223602A - Iron-silicon alloys and treatment thereof - Google Patents

Description

United States Patent 3,223,602 IRON-SILICON ALLOYS AND TREATMENT THEREOF Hans Werner Wawrousek, Pittsfield, Mass, assignor to genltzral Electric Company, a corporation of New or No Drawing. Filed May 17, 1961, Ser. No. 110,663 6 Claims. (Cl. 204-1405) The present invention relates to iron-silicon alloys, and more particularly to a process of treating iron-silicon alloys of silicon content higher than about 3 /z% to render them suitable for use in electromagnetic apparatus and to facilitate their processing for this purpose.

In the construction of cores for electromagnetic apparatus, such as transformers, it has been customary to use laminations of silicon steel alloy having a silicon content of about 3%%. Silicon steel containing more than that amount of silicon has not generally been used by the industry because the addition of silicon to iron or steel is well known to impart brittleness thereto, and increasingly so above 3% silicon, so that even in very thin sheets, electrical steels having a silicon content above about 5% are extremely difficult to handle without risk of breakage during the assembly procedures. On the other hand, it is known that the higher the silicon content, the lower will be the hysteresis loss, and the lower the coeflicient of magnetostriction. The latter factor provides an advantage in the use of higher silicon-content steel in that transformer noise due to magnetostriction is thereby greatly reduced.

It is an object of the invention to provide silicon steel material of the above type which has good ductility coupled with low coetlicient of magnetostriction and low hysteresis loss.

It is a particular object of the invention to provide a method of simply and economically improving the ductility of high silicon content electrical steel.

Other objects and advantages will become apparent from the following description and the appended claims.

With the above objects in view, the present invention relates to a method of making ductile, highly grain-oriented silicon steel containing about 3.5% to 5% silicon which comprises alternately rolling and heat treating such silicon steel sheet material for reducing it to the desired thickness and for developing the grain crystals therein in a high degree of directional orientation, and electrolytically polishing the thus processed silicon steel material. The electrolytic polishing step of the above-described process has been unexpectedly found to remarkably improve the ductility of the high silicon content grain-oriented silicon steel material.

In the usual processing of silicon steel strip to produce material of good magnetic and electrical properties, ingots of silicon-iron alloy containing the desired amount of silicon are hot-worked usually by rolling to less than 0.15" strips or sheets called band. These are then subjected to varying schedules of unidirectional cold rolling. The usual practice comprises cold-rolling the material in one or more stages from a thickness of about 0.10" to about 0.014" with intervening heat treatments, followed by a decarburizing heat treatment and a final stage texturedeveloping and purifying heat treatment at temperature in excess of 1100 C. in a hydrogen atmosphere. This latter heat treatment normally develops in the steel a crystal structure (texture) so oriented that good magnetic properties are obtained in the strip. Such crystal structure is developed by secondary recrystallization, as known in the art, and as described in detail, for example, in US. Patent 2,986,485. The final heat treatment also normally removes impurities such as sulfur and carbon which may cause excessive watt loss in the oriented strip. The preferred crystal orientation resulting from the above treatment which provides the desirable magnetic properties sought in the finally processed electrical silicon steel is conventionally designated in the art as a (110) [001] crystallographic orientation, the notation being in terms of Miller Indices.

When the silicon steel material contains more than about 4% silicon, and especially above 5% silicon, it is so brittle that it is readily subject to the risk of breakage during the subsequent handling of the processed steel.

I have found, however, in accordance with the invention that silicon steel containing up to about 5% silicon, after being processed in the usual way as described above, may be markedly improved in ductility simply by electrolytically polishing its surface. By this simple expedient, the quite brittle, fully processed silicon sheet is endowed with a bend ductility such as characterizes a 3.25% silicon steel alloy.

A particular electropolishing procedure which has been found satisfactory to obtain improved ductility comprises treating the silicon steel material in an electrolytic bath composed of a solution of phosphoric acid and a suitable oxidizing agent such as chromium trioxide (CrO In an experiment carried out in practicing the invention, Epstein strips of electrical steel alloy containing 3.8% silicon and fully processed to obtain optimum magnetic properties as above described were made the anode in an electrolyte bath composed of a solution of 100 grams chromium trioxide in 5 30 cc. of phosphoric acid, the cathode being a stainless steel member. The bath was at a temperature of C. and an electric current density of 2-3 amps/in. at a voltage of about 20 volts was applied for 1015 minutes. Following such treatment which resulted in a mirror surface on the samples, it was found that the Epstein strips had excellent ductility and could be bent into virtually any shape and at extreme angles without breaking. In contrast, Epstein strips identical to those described except that they had not been subjected to the electrolytic polishing treatment were found to fracture readily when bent through an angle of 180.

In the above electropolishing treatment, the bath temperature may be in the range of 70 to C., the current density in the range of 1 to 3 amps/in. and the time from about 10 to 20 minutes, while providing satisfactory results.

In another series of experiments, the same treatment was applied to similar, fully processed Epstein samples containing 5.2% silicon. Prior to the electropolishing step, these samples could not be bent at all without breaking. After the electropolishing treatment which produced a satin-like finish on their surfaces, samples could be bent up to without fracturing. Other experiments have shown that abrading the fully processed high silicon content alloy will not produce the improved ductility afforded by the present invention, and also show that treating the silicon steel material with pickling solutions and procedures known in the art likewise are not effective for this purpose.

The reason why electrolytic polishing in accordance with the invention provides such marked improvement in ductility in high silicon content steel alloys is not fully known. In part, such improvement may be attributable to removal by the polishing step of an oxide layer formed by the final annealing process which results in trapping hydrogen in the steel. However, mere removal of such a layer is apparently not sufficient in view of the demonstrated ineffectiveness of the aforementioned pickling or abrading steps. It appears possible that the nascent oxygen observed to be evolved at the anode reacts with the silicon steel material, and the elevated temperature generated by this and other reactions occurring at the anode results in liberating the hydrogen from the silicon steel and reduces the embrittling which it may cause. It should be understood, however, that I do not intend to be bound by the explanation thus presented, since other theories may very well apply to explain the results obtained. For example, it is conceivable that silicon steel material with over 3.5% silicon may be notch-sensitive, that is, readily susceptible to fracture by bending because of the presence on its surface of notches, scratches, or the like however small they may be, and the improved ductility imparted thereto might well result from the complete removal or smoothing of such surface irregularities by the electropolishing treatment described herein.

A typical procedure which may be employed in processing the silicon steel alloy prior to subjecting it to electropolishing in accordance with the invention is as follows, it being understood that the specific procedure is given by way of example only:

Raw iron scrap containing the usual proportion of impurities such as phosphorus, sulfur, chromium, nickel, aluminum, manganese, and copper is melted in a furnace with about 3.56% by weight of silicon (preferably about added thereto. With the temperature of the melt adjusted to about 1600 C., the mixture is poured into an ingot mold. After solidification, the ingot is hot rolled to a strip 100 mils thick, and then rolled to an intermediate gage, e.g., 30 mils. The strip is then heat treated in an open anneal, cooled and cold rolled to the desired final gage, e.g. 14 mils. The strip is then decarburized by heat treatment in a wet non-carburizing atmosphere at 800 C. Thereafter, the thus-treated strips are subjected to an anneal at about 1175 C. for about eight hours or a sufficient period to grow secondary crystals of optimum size and proper orientation and to further purify the strip by the removal of residues of carbon, sulfur, oxygen, and other impurities.

Following the above treatment, the thus fully processed steel is subjected to the electrolytic polishing treatment to improve its ductility in accordance with the invention, as described above.

There is thus provided in accordance with the invention, a simple, economical, and convenient process whereby it is possible to markedly improve the ductility of high silicon content electrical steel sheet material while obtaining the low hysteresis loss and the low magnetostriction which normally characterize such silicon steel material. In this way, substantial reduction in transformer noise can be accomplished, without sacrifice in good electrical properties.

The expressions sheet material and sheet as used in the appended claims are intended to include such forms as sheets, strips, tapes, and other laminar shapes.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making ductile highly grain oriented silicon steel containing from above about 4% to about 6% silicon which comprises alternately rolling and heat treating silicon steel sheet material containing from above about 4% to about 6% silicon for reducing the same to the desired thickness and developing the grain crystals therein in a high degree of directional orientation, and electrolytically polishing the thus-processed silicon steel material.

2. The method of making ductile highly grain oriented silicon steel containing from above about 4% to about 6% silicon which comprises alternately rolling and heat treating silicon steel sheet material containing from above about 4% to about 6% silicon for reducing the same to the desired thickness and developing the grain crystals therein in a high degree of directional orientation, and electrolytically polishing the thus-processed silicon steel material by making the silicon steel material the anode in an electrolytic bath containing phosphoric acid.

3. The method of making ductile highly grain oriented silicon steel containing from above about 4% to about 6% silicon which comprises alternately rolling and heat treating silicon steel sheet material containing from above about 4% to about 6% silicon for reducing the same to the desired thickness and developing the grain crystals therein in a high degree of directional orientation, and electrolytically polishing the thus-processed silicon steel material by making the silicon steel material the anode in an electrolytic bath containing phosphoric acid and an oxidizing agent and heated to a temperature of about to C.

4. The method as defined in claim 3, wherein said oxidizing agent comprises chromium trioxide.

5. The method of making ductile highly grain oriented silicon steel containing from above about 4% to about 6% silicon which comprises alternately rolling and heat treating silicon steel sheet material containing from above about 4% to about 6% silicon for reducing the same to the desired thickness and developing the grain crystals therein in a high degree of directional orientation, and electrolytically polishing the thus-processed silicon material by making the silicon steel material the anode'in an electrolytic bath containing phosphoric acid and an oxidizing agent and heated to a temperature of about 70 to 100 C., and applying a current to the bath sufiicient to produce a current density of 1 to 3 amperes/in. for a period of 10 to 20 minutes.

6. Electrical silicon steel having good ductility coupled with low coefficient of magnetostriction and low hysteresis loss comprising silicon-iron alloy sheet material containing from above about 4% to about 6% silicon having a high degree of grain orientation and made by the process as defined in claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,334,699 11/1943 Faust 204-140.5

2,347,040 4/ 1944 Faust 204140.5

2,366,712 1/1945 Faust 204140.5

2,424,674 7/1947 White 204140.5

2,986,485 5/1961 Fitz 148112 OTHER REFERENCES Jacquet, P.: Metal Finishing, February 1950, pages 55-60.

JOHN H. MACK, Primary Examiner.

Claims (1)

1. THE METHOD OF MAKING DUCTILE HIGHLY GRAIN ORIENTED SILICON STEEL CONTAINING FROM ABOVE ABOUT 4% TO ABOUT 6% SILICON WHICH COMPRISES ALTERNATELY ROLLING AND HEAT TREATING SILICON STEEL SHEET MATERIAL CONTAINING FROM ABOVE ABOUT 4% TO ABOUT 6% SILICON FOR REDUCING THE SAME TO THE DESIRED THICKNESS AND DEVELOPING THE GRAIN CRYSTALS THEREIN IN A HIGH DEGREE OF DIRECTIONAL ORIENTATION, AND ELECTROLYTICALLY POLISHING THE THUS-PROCESSED SILICON STEEL MATERIAL.