US3227587A - Method of annealing magnesia coated silicon-iron alloys in a vacuum - Google Patents

Description

United States Patent f 3,227,587 METHOD OF ANNEALING MAGNESIA COATED SILICON-IRGN ALLOYS IN A VACUUM Jack P. Martin, New Kensington, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsyivania N0 Drawin Filed Aug. 18, 1959, Ser. No. 834,420 5 Claims. (Cl. 148113) This invention relates to the production of magnetic materials and particularly to a new and novel heat treatment of silicon steel strip to develop preferred cube-onedge texture crystal orientation and the magnetic characteristics commonly associated with such strip.

In the maufacture of electrical grades of silicon steel strip for use in magnetic cores for electrical apparatus, such as large transformers and generators, it is common practice to produce a strip that has improved magnetic properties in one direction. Such a practice enables one to secure better magnetic properties, such as high penneability and low core loss, in the direction parallel to the rolling direction. Such a treatment is taught in United States Patent No. 1,965,559 to Goss.

Processing to develop maximum directional properties, in order to take advantage of this phenomenon, involves hot rolling of the steel to a gauge of from .05 to .10 inch, followed by cold rolling and continuous strand annealing treatments to reduce the strip to a gauge of from about .01 to .05 inch. The cold rolled continuous open annealed or normalized material does not exhibit the optimum directional magnetic properties. Primary recrystallization such as may occur during the continuous open anneal or heat treatments between the different cold rolling steps has no substantial effect on directional properties. To impart the desired texture, or what is commonly known as [100] (110) or cube-on-edge crystal orientation, with the [100] direction, i.e., a cube-edge parallel to the rolling direction, it is necessary to heat the strip to a temperature in exces of about 1400 F. for a sufiicient time to effect substantial secondary recrystallization where crystals of the preferred [100] (110) texture grow in preference to other crystals. To effect maximum directional properties, it is necessary to hold the strip in a temperature range of from about 1400 F. to 1800 F. for a sufficient time to effect a secondary grain growth of the preferred oriented crystals.

Small, but significant amounts of impurities, such as carbon, manganese and sulfur, are usually present in this steel since they cannot be economically removed or avoided in the melting process or because they are added for metallurgical control of the alloy. Carbon is picked up from the pig iron and other raw materials in the melt charge. Its subsequent removal to low levels with iron ore or oxygen during melting provides a source of heat and a means for accomplishing other refining processes in the melting furnace. Sulfur is generally undesirable at all levels and is controlled mainly by the use of low sulfur raw materials. The sulfur content can be further reduced during refining of the melt, but this process involves additional costs, and in some instances sulfur may be desirable in limited amounts, since the distribution of sulfide particles in the wrought alloy has a beneficial influence in the development of the desired texture in the grain oriented grades of silicon iron alloys. However, in any event, in the finalproduct, it is desirable to have the sulfur and carbon content of all the silicon iron alloys as low as possible for the best magnetic quality. Carbon may be removed during processing by exposing the surface of the thin sheet or strip to a decarburizing atmosphere at a tem- 3,227,587 Patented Jan. 4, 1966 perature sufiiciently high for carbon and oxygen to combine rapidly. Sulfur is not so easily removed and once the steel is cast the sulfur level cannot be substantially lowered except in very high temperature heat treatments. A reducing atmosphere (usually dry hydrogen) and basic refractory coatings are employed for this purpose in which the sulfur combines with the coating and/ or with the hydrogen and is removed as hydrogen sulfide. Other elements, such as carbon, oxygen, nitrogen and manganese, may also be lowered beneficially in the high temperature reducing atmosphere anneal.

The requirements for the complete processing of commercially preferred grain oriented grades of silicon steel strip to obtain optimum magnetic characteristics involve a single high temperature box anneal to a temperature range of from about 2050 F. to 2200 F. in a reducing atmosphere. It is conventional practice to box anneal coils of strip in a dry hydrogen atmosphere for periods of from 50 to hours at temperatures ranging from 1400 F. to 2200 F. In box annealing, conventional production size charges require considerable time to reach temperatures in excess of about 1800" F., and the heating cycle is such that coils o f the strip are held within the 1400 F. to 1800" F. temperature range for sufficient time to effect the cube-on-edge or crystal texture and are controlled within the range of 1800 F. to 2200 F. for sufficient time to remove sulfur and the other undesirable materials as well as cause the grain growth process. The total time of such heat treatment frequently is as long as 75 hours but may be as short as 1 hour.

Usually the strip is first given a continuous decarburizing heat treatment at about 1475" F. in a hydrogen-bearing atmosphere that contains sufiicient water vapor to provide a high dew point before final box annealing. Such treatment effects primary recrystallization but is not of suificient duration to cause the desired secondary'recrystallization and does not alter the necessity for the subsequent box anneal that is required to obtain substantially complete [100] (110) crystallization, grain growth and chemical purification. The decarburizing heat treatment may be employed in some other sequence than directly following the last cold rolling, the strip sometimes being decarburized prior to the first cold roll. Such a treatment may or may not be necessary depending on the carbon content of the strip and the ultimate use of the material.

It may also be desirable to short time anneal the silicon steel strip parts or magnetic cores that have been sheared or fabricated after final orientation and heat treatment to relieve stresses imparted to the material by such fabrication or shearing steps. Shear stress adversely affects the electrical and magnetic properties of the steel. Such a heat treatment usually involves heating the material to a temperature of about 1450" F.

In the past, the pure dry hydrogen has been thought to be the most satisfactory atmosphere for high temperature annealing. This atmosphere was believed not only to serve to purify the steel by removing sulfur (theoretically as (H S), oxygen, nitrogen and carbon, but contains practically no contaminants and the small amount of hydrogen which dissolves in this steel is not considered to be detrimental.

In performing the high temperature long time box anneal, it is necessary to employ coatings on the surface of the steel in order to avoid adjacent surfaces of the coiled metal from fusing together at such temperatures. The material most commonly employed is magnesia, wherein magnesia is applied to the surface of the silicon steel by passing the steel through aqueous suspensions of such material (at least one of M O, hydrous M O and M OH). Magnesium oxide coatings are highly satisfactory for this purpose; however, they react with the silicon of the steel during the high temperature heat treatment to form magnesium orthosilicate glass. The magnesium orthosilicate glass is a good base for subsequent electrical insulating coats; however, such a glass is highly detrimental when such steel is employed for stamping large quantities of laminations, as is frequently the case. The orthosilicate glass causes excessive wear on punches and dies. Present practices of preparing silicon steel strip to be punched into laminations (particularly .014 inch material) is to chemically pickle the orthosilicate glass from the surface of the strip. This procedure has proved to be highly unsatisfactory inasmuch as the resultant surface is rough and undesirable and such a pickling step is costly. The orthosilicate glass clings tenaciously to the steel and is much more difiicult to remove than ordinary iron oxide scale.

It has now been found that the final high temperature box anneal may be efiected, While reducing the sulfur content and other impurities to the desired level, without the formation of the magnesium orthosilicate glass that is so highly detrimental for subsequent stamping operations.

It is therefore the object of the present invention to provide a method of box annealing electrical grades of silicon steel strip within the temperature range of from 1400 F. to 2200 F. without the formation of a magnesium orthosilicate glass coating,

It is also an object of the present invention to provide a new method of annealing electrical grades of silicon steel strip, other than a high temperature reducing atmosphere box anneal.

Further objects and advantageous features of the present invention will be obvious from the following de tailed description:

In general, the present invention relates to the substitution of the dry hydrogen atmosphere of the conventional final box anneal with the process of effecting the final box anneal in a substantial vacuum.

It has been thought in the past that the use of a reducing atmosphere and particularly a hydrogen atmosphere was necessary for sulfur removal during the high temperature anneal of electrical grades of silicon steel strip inasmuch as it has been assumed that sulfur migrating from the steel during the anneal is necessarily removed from the surface of the steel as H 8 and that, therefore, hydrogen must be present to permit this reaction. However, it has now been determined that the final box anneal may be elfected in a substantial vacuum while substantial and signficant amounts of sulfur as Well as other undesirable impurities and residual elements are removed in a corollary manner.

It is a well known fact that the high temperature heat treatment causes excessive gain growth which though desirable for magnetic properties is undesirable from the standpoint of mechnical properties, particularly where subsequent forming steps of the metal are to be taken. Therefore, from the standpoint of desired mechanical properties, it would be highly desirable and may be the practice to effect the high temperature box anneal at some time prior to the final cold rolling and annealing cycle.

Although optimum magnetic properties may not be obtained in strip that has been subjected to the grain orientation and purification treatment prior to final cold rolling, such procedure may be followed to secure optimum mechanical properties. Such treatment would obviously have to be effected after final hot rolling because hot rolling after securing preferred orientation would alter the desired cube-on-edge structure. In any event, the advantages of the present invention would be as equally desirable and effective as when applied to such an anneal when it is performed as the concluding treatment of strip preparation. Coating to prevent fusion of adjacent metal surfaces during the high temperature box anneal is necessary regardless of the sequence of the processing steps. The removal of a glass presents an equal or greater problem in this event, because such a coating would have to be completely removed before subsequent cold rolling would be practical. By providing a substantial vacuum instead of hydrogen when performing the high temperature treatment prior to final cold rolling, the steel surface is economically and easily prepared for the subsequent fabrication.

Very thin oriented steels of 1-6 mils thickness are produced by the further cold rolling of full finished hydrogen annealed oriented steel in a range of thickness of .0075 to .025". Again the hard abrasive surface produced in the hydrogen anneal results in a costly and usually unsatisfactory stripping operation. The substitution of a vacuum anneal in place of the hydrogen anneal of the starting material offers a practical cost saving solution to the problem of surface preparation of these products.

The coating applied to the surface of the steel, to separate it and prevent welding of adjacent steel surfaces at high temperatures, contains considerable amounts of combined water since it is best applied as a Water slurry. The presence of moisture in the annealing atmosphere can effectively retard the sulfur removal process when this atmosphere is hydrogen. Annealing under vacuum will remove all moisture at a much faster rate so that it will not be present during the high temperature portion of the annealing cycle. To accomplish these conditions in a hydrogen anneal requires that the anneal be held at a low temperature for several hours to permit the moisture to be swept out by the hydrogen atmosphere after the coating has reached a temperature at which the chemically combined water is released to the atmosphere. Under vacuum this temperature will be much lower. The moisture would be removed at a faster rate since it is positively evacuated instead of being merely diluted with dry hydrogen.

The vacuum employed as a substitute for the dry hydrogen atmosphere in the high temperature box anneal of electrical grades of silicon steel strip may vary considerably during annealing since gas will be given off by the steel; however, such a vacuum must be maintained at a level that will avoid oxidation and that will, in fact, promote gaseous emission from the metal. Therefore, it is necessary that the vacuum be a substantial vacuum but, of course, may vary. For example, it would not be practical to carry out the process of the present invention where the pressure in the annealing furnace exceeded 1000 microns of mercury. It is preferred, however, to employ vacuums of 100 microns of mercury and less.

The following specific examples are given to illustrate the effects of the employment of the present invention and in no way limit the invention to the exact embodiments set forth in Table I. Table I shows the comparative results of annealing coiled samples of magnesia coating electrical grades of silicon steel strip in hydrogen atmosphere and in substantial vacuums ranging in pressures of from 60 to about 200 microns of mercury, the temperatures ranging from about 70 F. to about 2150 F. and were within the range of 1400 F. to 2150 F. for approximately 40 hours. At the conclusion of the testing, the separating medium (MgO) of the material subjected to the vacuum anneal was granular and loose with no apparent adhesion to the steel surface. It was readily removed in dilute sulfuric acid (10%). The steel subjected to the hydrogen atmosphere anneal exhibited a tenacious glass coating at the conclusion of the anneal which required repeated cycling through acid pickling baths.

Table I 60 cps. Magnetic Properties, and percent C, S, and

Mn after a Hydrogen Anneal at 2,150 F. of Grain Thick Oriented Silicon Steel Lot No. ness Inches Watts/lb. B at n at Pcr- Pcr- Per- 15000B 10H 200B cent cent S cent Mn Inches 532473 014 586 17, 940 8400 0067 004 039 53-2480- 014 619 17, 850 8260 0034 004 040 52-7248"- 014 827 16, 050 5800 0035 005 057 52-7206 012 552 18, 170 8770 0055 004 0-13 Z7194 012 649 16, 880 7550 0033 003 051 532573 012 717 16, 280 6150 0029 005 051 60 cps. Magnetic Properties and percent C, S, and Mn after a Vacuum Anneal at 2,150 F. of Grain Thick- Oriented Silicon Steel Lot No. ness,

Inches,

Watts/1b. B at a at Per- Per- Per- 1500013 H 200B cent cent cent 0 S Mn CHELHCAL ANALYSES OF SAMPLES AS CAST BEFORE ANNEALING 1. In the process of making magnetic strip material wherein silicon steel strip is coated with at least one material selected from the group consisting of MgO, anhydrous MgO and MgOH, coiled and box annealed within the temperature range of from 1800" -F. to 2200 F., the improvement comprising, conducting said box anneal in an atmospheric pressure of 1000 microns oi mercury or less.

2. In the process of making magnetic strip material wherein silicon steel strip is coated with at least one material selected from the group consisting of MgO, anhydrous MgO and MgOH, coiled and box annealed within the temperature range of from 1800 F. to 2200 F, the improvement comprising, conducting said box anneal in an atmospheric pressure of microns of mercury or less.

3. In the process of making magnetic strip material wherein silicon steel strip is coated with at least one material selected from the group consisting of MgO, anhydrous MgO and MgOH, coiled and box annealed at a temperature of at least 1800 F., the improvement comprising, conducting said box anneal in an atmospheric pressure of 1000 microns of mercury or less.

4. In the process of making magnetic strip material wherein silicon steel strip is coated With at least one material selected from the group consisting of MgO, anhydrous MgO and MgOH, coiled and box annealed at a temperature of at least 1800 F, the improvement comprising, conducting said box anneal an atmospheric pressure of 100 microns of mercury or less.

5. In the process of making magnetic strip material wherein silicon steel strip is coated With a magnesium oxide slurry, and box annealed at a temperature of at least 1800 F., the improvement comprising conducting said box anneal at a pressure not exceeding 0.20 micron of mercury.

References Cited by the Examiner UNITED STATES PATENTS 1,110,010 9/1914 Ruder 1481l3 1,973,525 9/1934 Dahl et a1. 148-113 2,389,497 11/ 1945 Gat 1481 13 2,535,420 12/1950 Jackson 148111 2,992,951 7/ 1961 Aspden 148-111 2,992,952 7/1961 Assmus et a1. 148-111 FOREIGN PATENTS 1,009,214 5/1954 Germany.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, MARCUS U. LYONS,

Examiners.

Claims (1)

1. IN THE PROCESS OF MAKING MAGNETIC STRIP MATERIAL WHEREIN SILICON STEEL STRIP IS COATED WITH AT LEAST ONE MATERIAL SELECTED FROM THE GROUP CONSISTING OF MGO, ANHYDROUS MGO AND MGOH, COILED AND BOX ANNEALED WITHIN THE TEMPERATURE RANGE OF FROM 1800*F. TO 2200* F., THE IMPROVEMENT COMPRISING, CONDUCTING SAID BOX ANNEAL IN AN ATMOSPHERIC PRESSURE OF 1000 MICRONS OF MERCURY OR LESS.