US3345219A - Method for producing magnetic sheets of silicon-iron alloys - Google Patents

Description

United States Patent O 3,345,219 METHOD FGR PRODUCING MAGNETIC SEEETS F SILICON-IRON ALLOYS Klaus Detert, Berlin-Zehlendorrf, Germany, assignor to Vacuumschmelze Aktiengesellschaft, Hanan, Germany, a corporation of Germany No Drawing. Filed May 4, 1960, Ser. No. 26,672

15 Claims. (Cl. 148-112) This application is a continuation-in-part of my application Ser. No. 796,281, filed Mar. 2, 1959, now abandoned, which latter application is based on the following patent application filed in Germany: V 14,058, filed Mar. 15, 1958.

This invention relates to a method for producing magnetic sheets of silicon-iron alloys.

It is known that many soft magnetic materials, such, for example, as silicon-iron and nickel-iron alloys which crystallize in the cubic system, are characterized by the direction of easiest magnetization being along the cube edge. The permeability values of these magnetic materials are higher in the direction of the cube edge than in any other direction. The prior art manufacturing practices have produced from silicon-iron alloys singly oriented or Goss texture magnetic sheet materials in which a majority of the grains are disposed on one edge parallel to the surface of the sheet and the cube edges of a high proportion of these grains are oriented in the direction of rolling of the sheet of the magnetic alloys. In Miller Indices, this is the (-110) [-001] orientation. Consequently, in the silicon-iron alloy sheets having such a single orientation grain texture, the permeability is greatest in the rolling direction of the sheet of the alloy. However, in a direction perpendicular or crosswise to the rolling direction, the grains or crystals of the alloy are not oriented with cube edges in the plane of the sheet. Consequently, substantially poorer magnetic properties are exhibited in directions crosswise to the rolling direction of the sheet.

In accordance with the present invention, a cube texture or double oriented grain structure is formed in silicon-iron sheets for the purpose of producing materials that have particularly favorable magnetic properties in two directions perpendicular with respect to each other. So far iron -silicon alloys have been used in electrical engineering which had either a random structure or a so-called Goss texture wherein a preferred magnetic direction extends in the direction of rolling, wherein, however, a magnetization transverse to the direction of rolling is difficult; Materials of this type have been primarily used for wound cores but were not satisfactory, for example, for the construction of cores for transformers, reactors or the like in which punched laminations are used. 'For ex ample, if cores are made from U-shaped laminations consisting of a material having Goss texture only the legs of U-laminations, but not the yokes thereof, will extend in the preferred direction, or vice versa. Therefore, if it is desired to utilize also in this case the favorable magnetic properties observed in the preferred direction, it is necessary to resort to a complicated assembly of the laminated cores or to put up with a considerable waste of material upon the punching of the laminations, or with magnetic losses at the joints of the laminations.

These drawbacks can be obviated by using for the assembly of cores iron-silicon alloys which have cube texture or double orientation, that is (100) [001].

An older disclosure (US. patent application Ser. No. 623,596, now abandoned, filed Nov. 21, 1956 and a continuation-in-part thereof, Ser. No. 706,103, filed Dec. 30, 1957, now US. Patent No. 2,992,952, both being based on Geman applications V 9825, V 10,655 and V 11,083) describing a method of obtaining a cube texture in iron- 3,345,219 Patented Oct. 3, 1967 silicon alloys, but not constituting prior art, describes the process steps of subjecting the alloys after a hot-working operation to one or several cold-working operations, the last cold-working step preferably resulting in a deformation of from 50% to and, as the case may be, to intermediate anneals at 750 C. to 950 C. and a final anneal at above 950 C., preferably between 1100 C. to 1350 C., such that at the final anneal the oxygen partial pressure of the annealing atmosphere directly at the surface of the article to be annealed is maintained at least so low that at annealing temperature, the annealing atmosphere will form no silicon oxide on the surface of the article to be annealed, and in fact that any silicon dioxide possibly present thereon will disappear. Also in accordance with said previous disclosure, the annealing period and the annealing temperature should be so selected with respect to each other and to the annealing atmosphere that the grain structure of the sheets will undergo second ary recrystallization and will result in a complete cube orientation of the grain structure of the sheets.

These dis-closures, which also do not constitute prior art, describe how the low oxygen partial pressure can be obtained, for example, by maskingor placing adjacent to the articles to be annealed at the final anneal getters or substances such as platinum, nickel, nickel alloys, cobalt and cobalt alloys, which have a catalytical effect on the conversion of molecular hydrogen into atomic hydrogen. Such atomic hydrogen will react with any oxides on the metal surface or in the gas to reduce the partial pressure of oxygen.

An object of the present invention is to provide a process that is particularly effective in producing an excellent cube texture grain structure in thick sheets of relatively pure iron-silicon alloys containing from 2% to 5% silicon by following a critical series of annealing and rolling operations.

A further object of the invention is to provide for producing sheets of silicon-iron alloys containing from 2% to 5% silicon and of a thickness of 40 mils to 8 mils and less, preferably from 0.2 to 0.4 mm., with a high proportion of grains having a cube texture by annealing relatively thick hot rolled strips of the relatively pure alloy under conditions to remove surface oxides, applying an initial cold rolling to effect a slight reduction in thickness, annealing the slightly reduced sheet in a reducing atmosphere and then applying at least one cold rolling step which effects a reduction of from 50% to with an intermediate anneal if more than one cold rolling step is applied, and a final annealing after the last cold rolling reduces the sheet to desired thickness, the final annealing being at a temperature of above 1l00 C. in an atmosphere capable of reducing silica and being for a time suflicient to effect substantially complete secondary recrystallization.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The alloy to be employed in the practice of the invention should be a relatively pure material. It should be as free as is reasonably possible of impurities which tend to form intergranular precipitates. In particular, nitrogen, oxygen, sulfur and carbon should not be higher than 0.01% each, preferably not higher than 0.005% each. Also, the alloy should be relatively free of impurities which in the presence of oxygen will form relatively insoluble and difiicultly decomposible oxides. Particular reference is had to aluminum and titanium, each of which should not be present in amounts exceeding about 0.02%. Good magnetic properties have been obtained in sheets of silicon-iron alloys in which both aluminum and titanium were each present in amounts of about 0.01%. Optimum magnetic quality will be obtained when the aluminum and titanium each do not exceed about 0.002%. The total impurities preferably should not be in excess of about 0.05%.

While manganese is often present in silicon-iron alloys in substantial amounts, for the purpose of this invention it is preferred that the manganese not exceed 0.2%.

The silicon content of the alloys may vary from about 2% to 5% and preferably be from 2% to 3%. The balance is iron. Good results have been obtained in the practice of the invention by employing electrolytic iron, and silicon of a high purity such that the total impurities in the latter do not exceed about 0.01%

Ingots of the alloy may be prepared by melting the relatively pure iron and silicon with the smallest permissible amount of manganese, if desired, under conditions that assure a minimum introduction of oxygen, nitrogen and other impurities. Vacuum melting furnaces or electric furnaces may be employed for this purpose. The molten metal is poured into a mold and solidified into an ingot of desired shape. In some cases the ingot may be cast under vacuum in order to remove any gases present therein in substantial amounts. In certain phases of the invention the ingots may be cast to produce a columnar structure by cooling the ingot uniformly from the bottom to the top while the sides are maintained at an elevated temperature to minimize solidification from the side walls. Typical procedures for carrying out this columnar solidification are disclosed in Belgian Patent 560,938.

The ingot of the relatively pure silicon-iron alloy is hot worked, preferably by rolling, to reduce it to a strip of a thickness of from 0.1 to 0.5 inch. If a columnar cast ingot is employed, it may be rolled longitudinally after the surface layers of the ingot have been removed since the metal at the surface layer ordinarily is not oriented satisfactorily.

The atmosphere applied to the ingot and the hot rolled slabs during the hot Working operations should be maintained so that the purity of the silicon-iron alloy is pre served or a very minimum of oxide is formed thereon.

If at the conclusion of the hot rolling the temperature of the hot rolled strip leaving the last stand is below 900 C., it is particularly necessary to subject the hot rolled strip to an annealing treatment. In any event it is preferable to subject the hot rolled strip to an annealing treatment in a reducing atmosphere at a temperature from about 750 C. to 950 C. If the surface of the hot rolled steel contains any appreciable amount of oxides the annealing atmosphere should be hydrogen or other strongly reducing gas. If the hot rolling was carried out under conditions which produced no significant amount of oxides then the annealing may be carried out in a protective inert atmosphere or other nonoxidizing atmosphere. The purpose of this anneal is to convert the hot rolled grain structure which consists of elongated crystals or grains, into an irregular, equiaxed grain structure.

After the annealing of the hot rolled strip it is pickled, for example, in hydrochloric acid or sulfuric acid, to remove completely any residue of the original rolling scale and reduced rolling scale and other surface blemishes. "After such pickling treatment the surface of the hot rolled strip should be clean and free from any surface oxides or flaws.

Following the pickling the hot rolled strip is subjected to a first cold rolling step to effect a reduction in thickness of from about to This mild cold rolling step is for the purpose of destroying any undesirable grain texture components that may be present. It has been found that this initial deformation is extremely critical in producing a structure transformaton favorable to the attainment of a desired cube texture during subsequent rolling and annealing treatments.

Following the first mild cold rolling step, an anneal is applied to the strip at a temperature of at least 1100 C. to as high as 1350 C. This anneal is carried out in a reducing atmosphere. Hydrogen with a dew point of 30 C. or less is quite suitable. It should be under stood that hydrogen with a dew point of 30 C. indicates a hydrogen gas whose total oxygen content and water vapor content are so low that when the oxygen is converted into water the dew point of the gas does not exceed 30 C. This annealing may be performed in a vacuum of an absolute pressure of about 10" mm. mercury and less. The time and temperature of the anneal should be so correlated that the grains will recrystallize until the mean grain radius is at least half and may reach the thickness of the strip. At 1200 C. the annealing may take of the order of from 2 to 10 hours. Much shorter times are required at temperatures of 1300" C.

An examination of the strip after this anneal provides an indication of the suitability of the material for further treatment. If it is found that the anneal is not sufficient to develop a coarse grain structure then this indicates that the alloy contains an excessive amount of impurities. In this case it would not be expedient to process the material further in the expectation that it will haev a high proportion of cube texture.

Thereafter the annealed strip is subjected to at least one cold rolling step which will deform the sheet to reduce the thickness from 50% to and preferably from 70% to 80% for all but the final cold rolling step. The final cold rolling step should reduce the sheet from 50% to 80% and preferably 60% to 70%. It will be understood that by a cold rolling step it is intended to include a series of passes without any intervening annealing. Furthermore, the cold rolling which may be applied to the sheet at room temperature may increase its temperature due to the work performed thereon to several hundred degrees Centigrade and possibly up to 400 C.

If two or more of such cold rolling steps are applied, the sheet is subjected to an intermediate anneal between steps at temperatures of from about 700 C. to as much as 1000 C. or even slightly higher. The atmosphere during intermediate annealing should be a reducing atmosphere of such nature that no silicon oxides will be formed on the surface of the sheets and if possible sufiiciently reducing to cause silicon oxide present thereon to disappear. Either hydrogen of a dew point of 30 C. or lower or a vacuum at a pressure of not in excess of 10* mm. mercury are examples of suitable atmospheres.

After the final cold rolling step, which reduces the sheets to desired gauge, for example, less than 40 mils thickness and preferably to from about 0.2 to 0.4 mm., the sheets are subjected to a final anneal. During the early stages of the final anneal primary recrystallization occurs and the sheets will be found to contain a substantial proportion of primary grains with cube orientation. Thus, it has been found that more than 30% of the area of the surface comprises grains already doubly oriented. In order to complete a transformation of the structure of the sheets into a higher proportion of cube orientation texture, the sheet is subjected to a prolonged final anneal to effect substantially complete secondary recrystallization. The final anneal has for its object to cause substantially complete secondary recrystallization of the grain texture into a high proportion of cube-on-face grains. Ordinarily a grain coarsening action takes place during the anneal whereby the grains become so large that the average grain diameter usually equals the thickness of the sheet.

The final anneal is carried out under conditions that will bring about a selective growth of the grains having the [001] orientation which will consume grains of other orientations in the sheet. This can occur only if the annealing conditions are such that a substantially pure metal surface is present and the surface energy conditions favor the (l00)[001] grain growth. Films of oxides on the surface of the sheet, particularly films of silicon dioxide, impede this selective grain growth. Also, even small amounts of impurities in the sheet and on its surface disturb the selective growth of the grains. It seems that the selective growth of the cube grains is controlled by surface energy effects. The selective growth of the cube texture grains may be regarded as a new type of secondary recrystallization.

In order to produce a satisfactory textural transformation to obtain as distinct as possible a cube orientation at the final anneal it is important to maintain the impurity content of the silicon-iron alloy within the limits indicated herein. Furthermore, the annealing atmosphere should be sufiiciently pure to avoid any contamination of the surface of the sheet which would prevent or hinder the textural transformation into the double orientation.

The final annealing atmosphere should be one that will cause the reduction of silica. In order to accomplish this it is not necessary to reduce the oxygen partial pressure atmosphere) to the point where silicon dioxide will decompose. It is known that silicon metal in the sheets will react with silicon dioxide at elevated temperatures to form silicon monoxide. Silicon monoxide will evaporate at temperatures of 1100 C. and higher in atmospheres having a low oxygen partial pressure. Therefore, if a sufficiently pure annealing atmosphere and a sufficiently high temperature are employed silicon oxides will be converted to silicon monoxide and the latter removed from the surface of the sheet. It is not enough, however, that the requisite degree of purity of the annealing atmosphere is taken as an average throughout the annealing chamber. It is necessary that the requisite degree of purity exists also in the gas in direct contact with the surface of the sheets being annealed For the final anneal the temperature is at least 1100 C. up to as much as 1350 0., preferably between 1200 C. and 1300" C. For the final annealing, at least during the preliminary phases, until primary recrystallization is essentially completed and secondary recrystallization is initiated, dry hydrogen having a dew point of 50 C. and as low as 70 C., may be employed. Good results also are obtained in an atmosphere comprising a vacuum in which the pressure is at least 10* mm. mercury and preferably from 103 to 10-5 mm. mercury.

On carrying out the final anneal in vacuum metallic shields of pure iron or ferrous alloys, such as stainless steel and other iron-nickel alloys have proved advantageous since such shields act as getters. When interposed between the walls of the furnace and the sheet material, the shields absorb foreign substances originating in the walls of the furnace which usually are ceramic materials, the foreign substances being capable of effecting harmfully the atmosphere adjacent the surfaces of the sheet being annealed. Thus, compounds of sulfur and carbon are intercepted by such shields. The shields, particularly of platinum and platinum alloys, iron-nickel and ironcobalt alloys or pure nickel or cobalt metal, appear to support the conversion of molecular hydrogen into atomic hydrogen and thereby enhance reactivity of hydrogen with oxides on the sheet surfaces.

A very pure atmosphere for the final anneal may be assured by the use of a double-walled annealing furnace apparatus, for example, one comprising an internal vessel and an external vessel spaced therefrom. The sheet silicon-iron being annealed is disposed within the internal vessel. The space between the two vessels is filled with hydrogen having a dew point of less than 30 C. The internal vessel has a pressure of at least 0.1 mm. mercury less than the hydrogen between the two containers. In such a system the internal vessel may be evacuated to an initial pressure of 10*- mm. mercury. Hydrogen will slowly diffuse from the space between the walls of the two vessels into the internal annealing chamber until the vacuum in the inner vessel after several hours operation reaches a value of 0.1 mm. mercury. Hydrogen with a dew point of 30 C. at this low pressure does not introduce sufiicient oxygen or moisture to be detrimental to the annealing operation.

It will be understood that when the term sheet is employed herein, it refers not only to the sheet silicon-iron alloy in the form of coils or a stack of flat sheets but also applies to punchings or laminations of the sheet. In many instances, it is desirable to slit, punch or cut the final cold rolled sheets into desired shape and to apply the final anneal to such shape. The final cold rolled sheets may be given the final anneal by a conveyorized method. For example, a conveyor-type of annealing furnace may have inner and outer zones. The inner zone of the furnace is continually flushed with very dry hydrogen of a dew point which does not exceed 50" C. while the outer zones through which the conveyor carrying the annealed sheet passes and leaves to reach the final zone is flushed continually with hydrogen having a dew point of not over 30 C. The temperatures of the inner zone are 1100 C. and higher While the outer zones may be heated to a lower temperature. The outer zones of the furnace prevent oxygen from diffusing into the inner zone.

It will be understood that the magnetic sheets during annealing usually will be disposed in a stack or in a coil wherein successive sheets or turns of a coil are separated from each other by a porous refractory coating such as magnesium oxide, aluminum oxide, zirconium oxide or the like. The refractory oxide used must be so treated, as by previous firing at 1000" C. to 1300 C., as to be completely free from water and reactive materials and will form no reaction products with the silicon-iron sheets.

During the final anneal the silicon-iron sheets upon reaching the annealing temperature and being maintained there for the requisite period of time will be substantially completely secondarily recrystallized. The surface of the sheet will be bright, usually mirror bright, at the conclusion of the anneal. The secondary recrystallization texture of the silicon-iron will be cube texture or double oriented grain structure. The following examples illustrate the practice of the invention:

Example I An ingot was prepared by melting pure electrolytic iron having a total impurity content of 0.02% and pure silicon free from aluminum. The iron and silicon were combined in proportions to give a silicon content of 2.7% and melted in a vacuum of 5 1O- mm. mercury pressure. An ingot prepared from this melt was heated to 1200 C. and rolled into a strip of a thickness of 6 mm. After annealing the strip for three hours at 800 C. in hydrogen of a dew point of 30 C. the annealed strip was cooled and then pickled in hydrochloric acid in order to remove all blemishes from the surface. Thereafter, the strip was given an initial cold rolling to reduce it to a thickness of 5 mm. (approximately 17% reduction) after which it was annealed for five hours at 1200 C. in hydrogen having a dew point of -40 C. The annealed strip was found to be relatively coarse grained in which substantially all the grains had a diameter of from about 7 to 10 mm.

Thereafter, the strip was subjected to two cold rolling steps. The first step reduced the thickness to 1 mm. and was followed by an anneal in dry hydrogen of a dew point of 40" C. at a temperature of 900 C. for a period of three hours. The second and final cold rolling step resulted in a sheet of a thickness of 0.3 mm. The final anneal of this sheet was carried out in a vacuum at an absolute pressure of 5 X 10- mm. mercury at a temperature of 1200 C. The duration of the final anneal was eight hours.

The finally annealed sheet of a thickness of 0.3 mm.

was examined and found to have an excellent cube texture. All of the secondary recrystallized cube-on-face grains had an angular divergence of their faces of less than 5 with respect to the surface of the sheet. The alignment of the cube edges was such that the edges of more than 50% of the grains deviated from the rolling direction less than 5, more than of the grains had cube edges deviating from the rolling direction by less than 10 and the edges of practically all of the cubes deviated less than 20 from the rolling direction.

When tested magnetically the sheet exhibited an induction of 18.5 kilogauss at a field strength of 10 oersteds, both in the longitudinal and transverse directions of the sheet. The coercive force amounted to 70 millioersteds in both directions.

Example H An iron-silicon alloy with a content of 2.3% silicon was obtained by melting at a vacuum of 0.5 mm. mercury, a mixture of pure electrolytic iron with a total impurity content of 0.05% and pure silicon free from aluminum. After preparing an ingot of the alloy the casting surface was removed by machining and the ingot was annealed at 1200 C. after which it was hot rolled into a strip of a thickness of 6 mm. The resulting strip was annealed for three hours at 800 C. in hydrogen of a dew point of less than 30 C. The annealed strip was then pickled to remove rolling scale and any reduced rolling scale and other loose surface blemishes.

The pickled strip was then cold rolled to a thickness of mm. followed by an anneal at 1300 C. in hydrogen having a dew point of -40 C., for a period of five hours. Substantially all of the grains of the annealed sheet had a grain size of from 7 to mm. Thereafter, the strip was cold rolled to a thickness of 1 mm. (80% reduction) and annealed for three hours at 900 C. in hydrogen of a dew point of less than 40 C. The surface of the strip was lightly sanded to remove any surface defects. The sheet was then given a final cold rolling to reduce it to a final thickness of 0.3 mm. Following this the sheet was given a final anneal for eight hours at a temperature of 1200 C. For the first several hours the sheet was maintained under a vacuum of 10 min. mercury after which hydrogen of a dew point of 40 C. was slowly added in an amount to effect a final pressure of 0.1 mm. mercury.

The finally annealed strip was carefully examined and found to have an excellent cube texture. The angular divergence of the surface of the cube grains and the divergence of the cube edges from the direction of rolling were substantially equivalent to those of Example I.

When tested magnetically an induction of 18.3 kilogauss was obtained in both the longitudinal and crosswise direction of the sheet and a field strength of 10 oersteds. The coercive force amounted to 70 millioersteds in both directions.

Example 111 An ingot, designated as M103, of the following composition was cast from a melt in a crucible of pure alumina: 2.92% silicon, 0.0024% carbon, 0.003% sulfur, 0.0005% phosphorus, 0.002% manganese, 0.0022% nitrogen, less than 0.01% oxygen, 0.008% aluminum and less than 0.01% titanium, the balance being iron. The ingot was heated to 1300 C. and hot rolled into a band of 0.215 inch thickness. The band was annealed for two hours at 900 C. in dry hydrogen (less than 30 C. dew point). After pickling in acid the band was cold rolled in two passes to 0.175 inch thickness. The cold rolled sheet was vacuum (less than 10 mm. pressure) annealed at 1200 C. for four hours. The annealed plate was then cold rolled in five passes to a strip of a thickness of 0.035 inch, the successive strip thicknesses being 0.175; 0.136; 0.101; 0.073; 0.050; 0.035 inch, respectively.

The 0.035 inch thick strip was annealed for two hours at 900 C. in dry hydrogen (dew point below 50 C.). The strip was cold rolled in four stages to a sheet of a final gauge thickness of 0.011 inch, the successive thicknesses being as follows: 0.035; 0.0295; 0.022; 0.012; 0.011 inch, respectively.

Portions of the cold rolled 0.011 inch sheet were stacked with pure calcined alumina between successive layers and the stack was given a final anneal at 1200 C. for four hours. A vacuum of 10 to 5 10 mm. of Hg was employed. Substantially complete secondary recrystallization was obtained. From to of the volume of the strip comprised cube-on-face grains, that is, with cube faces within 5 of the plane of the sheet and cube edges within 15 of the rolling direction.

Magnetic tests of the 0.011 inch thick sheets of the M103 alloy gave the following results:

Coercive forces (H 0.073 Maximum permeability (,u 37,000 Induction in gauss at z B =l 12,700 B =5 15,700 B =10 16,600

1 Measured parallel to rolling direction.

The induction perpendicular to the rolling direction was similar to the values observed parallel to the rolling di rection.

Cores made of the M103 alloy sheets showed 60 cycles losses of 0.38 watt/lb. at an induction of 11,000 gauss, and 0.75 watt/lb. at an induction of 15,000 gauss.

Example IV An alloy identified as M134 was processed as in Example III. The analysis of the ingot gave the following composition: 2.99% silicon; 0.002% carbon; 0.0006% sulfur; 0.0005% phosphorus; 0.002% manganese, 0.0002% nitrogen; less than 0.01% oxygen; 0.01% aluminum; and 0.006% titanium, the balance being iron.

The finally annealed sheets of a thickness of 0.011 inch had essentially of their area of secondary recrystallized cube-on-face, (100) [001], grains.

Primary recrystallization and secondary recrystallization of silicon-iron alloys are described in the following two articles by C. G. Dunn and in the references listed therein: Acta Metallurgica, vol. 2, No. 2, March 1954, pages 174 to 183 and Acta Metallurgica, vol. 2, No. 3, May 1954, pages 386 to 393.

Briefly, primary recrystallization is the initial crystal texture that forms or grows out of the deformed grain texture of a previously worked body of metal. Thus, a cold rolled sheet of silicon-iron presents a more or less severely deformed grain texture, and upon subjecting the cold rolled sheet to annealing, first, stress relief occurs rapidly, usually with little change in grain texture, then concurrently with stress relief or soon thereafter, recrystallization occurs in which a different grain texture, designated as primary recrystallized texture, develops by absorbing the deformed texture. This primary recrystallized texture differs both in grain size and orientation of the grains from the deformed grain texture. For silicon steel primary recrystallization may begin at temperatures of 500 C., the rate being dependent on the temperature as well as the severity of the preceding cold rolling step.

If the temperature of the anneal is high enough, usually above 900 C., the primary recrystallized texture will be ordinarily converted to grains of a different orientation than that of the primary grains. This is secondary recrystallization. In some cases secondary recrystallization is initiated before primary recrystallization is completed. The secondary grains are almost invariably much greater in size than the primary grains. The rate and extent of growth of the secondary grains is strongly dependent on the temperatureusually temperatures of 1100 C. and higher are required in silicon steel to secure a high proportion of conversion of primary to secondary grains. The particular orientation that predominates in secondary grain growth in silicon steel is dependent on the composition of the silicon-iron alloy, impurities and additives being critical, the annealing furnace atmosphere, the primary grain orientation, and other factors.

It will be understood that the above description is illustrative and not limiting.

I claim as my invention:

1. In the process of producing a magnetic sheet having a high percentage of cube texture grain structure, the steps comprising hot rolling to a strip of a thickness of the order of 0.1 to 0.5 inch an ingot of an alloy composed of from 2% to of silicon, less than about 0.01% each of nitrogen, oxygen, sulfur and carbon, less than about 0.02% each of aluminum and titanium, not in excess of about 0.2% manganese, the balance being iron, annealing the hot rolled strip in a nonoxidizing atmosphere at a temperature of from 750 C. to 950 C., initially cold rolling the annealed strip to effect a. reduction of from about to 20%, annealing the resulting cold rolled strip at a temperature above 1100 C. in a reducing atmosphere for a period of time sufficient to cause grain growth such that the mean radius of the grains is at least half the thickness of the strip, cold rolling the strip at least once to effect a reduction at each cold rolling step of from 50% to 90%, applying an intermediate anneal to the resulting cold rolled sheet in the event more than one cold rolling step is applied, the intermediate anneal being at a temperature of from about 700 C. to 1000 C. in a reducing atmosphere, the final cold rolling step effecting a reduction of from 50 to 80% and producing a sheet of a final thickness of not in excess of 40 mils, and subjecting the final sheet to a final anneal at a temperature of at least 1100 C. in an atmosphere capable of reducing silica, the final anneal being for a period of time suflicient to enable substantially complete secondary recrystallization of the metal, the atmosphere at the surface of the sheet being of such low oxygen content that at the annealing temperature silica will disappear before secondary recrystallization occurs, and any refractory separator coatings on the sheet being non-oxidizing whereby the sheet surface is bright, with a major proportion of the secondary grains being doubly oriented.

2. The process of claim 1, wherein the hot rolled strip is pickled after the annealing thereof in order to remove surface defects.

3. The process of claim 1 wherein any of the annealing atmospheres, with the exception of the final atmosphere, comprise dry hydrogen of a dew point of at least 30 C.

4. The process of claim 1 wherein any of the annealing atmospheres, with the exception of the final anneal atmosphere, comprise a vacuum at a pressure not in excess of 10 mm. of mercury.

S. The process of claim 1 wherein the final anneal atmosphere is selected from the group consisting of hydrogen of a dew point of at least 40 C. and a vacuum of at least 10- mm. of mercury.

6. The process of claim 1 wherein the last cold rolling step effects a reduction of from 60% to 70%.

7. The process of claim 1 wherein each cold rolling step, except the first and the final cold rolling steps, effect a reduction of from 70% to 80% 8. The process of claim 1 wherein the intermediate anneal preceding the final cold rolling step is at a temperature of from 850 C. to 1000 C.

9. The process of claim 1 wherein the ingot has been cast to produce a columnar. grain structure and the hot rolling is performed in the direction of the columnar grains.

10. The process of claim 1, wherein the final anneal is at a temperature of from 1200" C. to 1300 C.

11. The process of claim 1, wherein the sheet during final anneal has disposed closely adjacent thereto a body of a metal selected from the group consisting of iron and nickel-iron alloys.

12. The process of claim 1, wherein the maximum amount of titanium and aluminum each does not exceed 0.002%.

13. In the process of producing a magnetic sheet having a high proportion of cube texture grain structure, the steps comprising hot rolling to a strip of a thickness of 0.1 to 0.5 inch an ingot of an alloy having from about 2% to 3% silicon, impurities not exceeding a total of about 0.02%, with less than about 0.02% each of aluminum and titanium being present, and the balance being iron, annealing the hot rolled strip in a dry hydrogen atmosphere for a period of at least one hour, pickling the hot rolled strip to remove surface blemishes, cold rolling the pickled strip to effect a reduction in thickness of from about 10% to 20%, annealing the cold rolled strip in hydrogen having a dew point of at least 30 C. at a temperature of from 1100 C. to 1350 C. for at least an hour until the mean grain radius is at least half of and approaches the thickness of the strip, applying an intermediate cold rolling step to the strip at least once to effect a reduction in thickness of from about to during each step, annealing the resulting sheet after each of said intermediate cold rolling steps at a temperature of from 850 C. to 1000 C. in hydrogen having a dew point of at least 30" C. for a period of time of the order of an hour, subjecting the sheet to a final cold rolling to effect a reduction in thickness of from about 60% to 70% to produce a sheet of a thickness of about 0.2 to 0.4 millimeter, and subjecting the final sheet to a final anneal at a temperature of from 1100 C. to 1350 C. in an atmosphere capable of reducing silica, for a period of time to produce substantially complete secondary recrystallization, the atmosphere at the surface of the sheet being of such low oxygen content that at the annealing temperature silica will disappear before secondary recrystallization occurs, and any refractory separator coatings on the sheet being non-oxidizing whereby the sheet surface is bright, the resulting annealed sheet having a major portion of its area comprised of doubly oriented grains.

14. The process of claim 13, wherein the atmosphere during the final anneal comprises hydrogen of a dew point of at least 50 C. during the initial stages of the anneal.

15. The process of claim 13, wherein the atmosphere during the final anneal comprises a vacuum of at least 10- mm. of mercury during at least the initial stages of the anneal.

References Cited UNITED STATES PATENTS 1,965,559 7/1934 Goss 148111 2,067,036 1/1937 Wimmer 148--111 2,076,383 4/ 1937 Pawlek et a1 148-111 2,940,881 6/ 1960 Hollomon 148-111 2,992,952 7/ 1961 Assmus et al. 148-1 11 3,008,856 11/1961 Mobius 1481 11 FOREIGN PATENTS 1,009,214 6/ 1957 Germany.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, MARCUS U. LYONS, WINSTON A. DOUGLAS, Examiners.

T. P. SCHILLER, W. B. NOLL, D. L. REISDORF,

N, MARKVA, Assistant Examiners.

Claims (1)

1. IN THE PROCESS OF PRODUCING A MAGNETIC SHEET HAVING A HIGH PERCENTAGE OF CUBE TEXTURE GRAIN STRUCTURE, THE STEPS COMPRISING HOT ROLLING TO A STRIP OF A THICKNESS OF THE ORDER OF 0.1 TO 0.5 INCH AN INGOT OF AN ALLOY COMPOSED OF FROM 2% TO 5% OF SILICON, LESS THAN ABOUT 0.01% EACH OF NITROGEN, OXYGEN, SULFUR AND CARBON, LESS THAN ABOUT 0.02% EACH OF ALUMINUM AND TITANIUM, NOT IN EXCESS OF ABOUT 0.2 MANGANESE, THE BALANCE BEING IRON, ANNEALING THE HOT ROLLED STRIP IN A NONOXIDIZING ATMOSPHERE AT A TEMPERATURE OF FROM 750*C. TO 950*C., INITIALLY COLD ROLLING THE ANNEALED STRIP TO EFFECT A REDUCTION OF FROM ABOUT 10% TO 20%, ANNEALING THE RESULTING COLD ROLLED STRIP AT A TEMPERATURE ABOVE 1100*C. IN A REDUCING ATMOSPHERE FOR A PERIOD OF TIME SUFFICIENT TO CAUSE GRAIN GROWTH SUCH THAT THE MEAN RADIUS OF THE GRAINS IS AT LEAST HALF THE THICKNESS OF THE STRIP, COLD ROLLING THE STRIP AT LEAST ONCE TO EFFECT A REDUCTION AT EACH COLD ROLLING STEP OF FROM 50% TO 90%, APPLYING AN INTERMEDIATE ANNEAL TO THE RESULTING COLD ROLLED SHEET IN THE EVENT MORE THAN ONE COLD ROLLING STEPS IS APPLIED, THE INTERMEDIATE ANNEAL BEING AT A TEMPERATURE OF FROM ABOUT 700*C. TO 1000* C. IN A REDUCING ATMOSPHERE, THE FINAL COLD ROLLING STEP EFFECTING A REDUCTION OF FROM 50 TO 80% AND PRODUCING A SHEET OF A FINAL THICKNESS OF NOT IN EXCESS OF 40 MILS, AND SUBJECTING THE FINAL SHEET TO A FINAL ANNEAL AT A TEMPERATURE OF AT LEAST 1100*C. IN AN ATMOSPHERE CAPABLE OF REDUCING SILICA, THE FINAL ANNEAL BEING FOR A PERIOD OF TIME SUFFICIENT TO ENABLE SUBSTANTIALLY COMPLETE SECONDARY RECRYSTALLIZATION OF THE METAL, THE ATMOSPHERE AT THE SURFACE OF THE SHEET BEING OF SUCH LOW OXYGEN CONTENT THAT AT THE ANNEALING TEMPERATURE SILICA WILL DISAPPEAR BEFORE SECONDARY RECRYSTALLIZATION OCCURS, AND ANY REFRACTORY SEPARATOR COATINGS ON THE SHEET BEING NON-OXIDIZING WHEREBY THE SHEET SURFACE IS BRIGHT, WITH A MAJOR PROPORTION OF THE SECONDARY GRAINS BEING DOUBLY ORIENTED.