US3337373A

US3337373A - Doubly oriented cube-on-face magnetic sheet containing chromium

Info

Publication number: US3337373A
Application number: US573641A
Authority: US
Inventors: Foster Karl; Paul A Albert
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1966-08-19
Filing date: 1966-08-19
Publication date: 1967-08-22
Anticipated expiration: 1984-08-22
Also published as: GB1141321A; SE313587B

Description

22, 1967 K. FOSTER ETAL 3,337,373

DOUBLY ORIENTED CUBE-ONFACE MAGNETIC SHEET CONTAINING CHROMIUM Filed Aug. 19, 1966 2 Sheets-Sheet 1 FIGJ.

If z, & V

INVENTORS Karl Foster and Paul A. Alberi ATTO EY 22, 1967 K. FOSTER ETAL 3,337,373

DOUB LY ORIENTED CUBE-ON-FACE MAGNETIC SHEET CONTAINING CHROMIUM Filed Aug. 19, 1966 2 Sheets-Sheet 2 2 FIGS.

o O I l l l WEIGHT "/0 ADDITION TO 3% Si-Fe O 65 Si 5 F IG.4.

WEIGHT ADDITION TO 3% Si-Fe DRECTION F IG. 7

United States Patent 3,337,373 DOUBLY ORIENTED CUBE-ON-FACE MAGNETIC SHEET CONTAINING CHROMIUM Karl Foster, Pittsburgh, Pa., and Paul A. Albert, Redford Village, N.Y., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 19, 1966, Ser. No. 573,641 7 Claims. (Cl. 148-3155) ABSTRACT OF THE DISCLOSURE Cold worked and annealed magnetic sheets having a major proportional of the volume of (100) [001] grains comprise an alloy of from 2% to 5% silicon and from 0.1% to 5% chromium, up to 0.4% manganese, balance 1ron.

This application is a continuation-in-part of our application Ser. No. 428,813, filed Jan. 28, 1965, now US. Patent 3,278,348, and applications Ser. No. 722,778, filed Mar. 20, 1958, and Ser. No. 295,264, filed June 21, 1963, both the last abandoned.

The invention relates generally to doubly oriented magnetic sheet having cube-on-face grain texture orientation.

It has been known for many years that the body-centered cubic crystals of magnetic material such as iron, iron silicon, iron chromium, iron molybdenum and other iron alloys have the highest magnetic permeability in a direction parallel to the cube edge of the crystal lattice of such grains. Processes for orienting the crystals of magnetic material in one direction have been known for many years. Goss US. Patent 1,965,599 taught a process for producing oriented magnetic material having a crystalline structure which is known as the Goss texture or cube-onedge texture. In the process taught in the Goss patent, a high percentage of the grains are oriented with the cube edge of the crystal lattice of the grains in one direction in the plane of the sheet and in the direction of rolling while four cube planes are at an angle of about 45 to the surface of the sheet. In the Miller Indices this grain orientation is the (110) [001] orientation. Other processes also have been disclosed and patented for orienting the cube edges of the grains in magnetic sheet in one direction.

When magnetic sheet is given a preferred orientation, that is, a high percentage of the grains are so aligned that the permeability in the preferred direction is much higher than when randomly oriented, it enables a great increase in the flux density at which the magnetic material may be worked when embodied in electrical apparatus. Immediately following the introduction of the orienting processes, attention was focussed on taking full advantage of the magnetic sheet with a single direction of preferred orientation in the manufacture of inductive apparatus such as transformers. This resulted in wound cores and specially designed laminations and core structures for three-phase transformers and other inductive apparatus. For power transformers and other inductive apparatus in which wound cores could not be used, specially designed laminations, such as employed in the well-known D-cores,'were devised for taking the best possible advantage of the preferred orientation of the magnetic sheet.

The introduction of magnetic sheet with a preferred orientation in one direction was widely accepted among builders of electrical equipment. It enabled them, by taking advantage of the high permeability of the magnetic sheet in the direction of rolling, to build certain types of apparatus, of a given rating, that required a much smaller weight of both magnetic iron and copper than was possible with the magnetic sheet available theretofore. In some instances, the weight of oriented magnetic ICC iron and copper required to build units of specified ratings was reduced to about 60% of the weight of the hot rolled non-oriented magnetic sheet and. copper previously employed.

The preferred orientation of magnetic sheet in only one direction, while it is a great advance in the art, has some shortcomings. While magnetic sheet with preferred orientation in only one direction had high permeability in one direction, the permeability at right angles to the preferred direction, being directed along the diagonal of the cubic crystal lattice, was lower than in randomly oriented material. This greatly limited the use of the cube-on-edge grain texture sheets where magnetic flux has to pass in a direction transverse to the preferred direction of orientation.

Doubly oriented magnetic material comprises sheets in which the crystal lattices of the grains comprise two cube faces parallel to the plane of the sheet and two edges of each of these cube faces are substantially parallel to the direction of rolling or the edge of the sheet, while two other edges of each of these two cube faces are perpendicular to the direction of rolling in the plane of the sheet. In Miller Indices this is the 100) [001]] grain orientation. Such doubly oriented sheets, or cube-on-face grain oriented sheets, provide improved magnetic sheet for the building of electrical apparatus of many types because the easiest direction of magnetization is both in the rolling direction and a direction transverse thereto.

The problems of working silicon-iron alloys increases with the silicon content. Up to about 3.5% silicon content, the industry has cold rolled silicon steel without great difliculty. With care, silicon steel having more than 3.5% silicon, up to 4.5% to 5% silicon, can be cold rolled satisfactorily to thin gauge sheets of 5 to 15 mils thickness, though the degree of cold reduction between intermediate anneals is much less so that the cost of producing the sheets is much greater than for the same gauge 3% silicon steel sheets. Beyond a 5% silicon content cold rolling of the steel to thin gauges is a formidable and costly problem. Much scrap and defective or poor quality sheets results. From a practical standpoint a 6% silicon iron sheet is not commercially cold workable. However, a 6% silicon steel is attractive from the standpoint of having a very high electrical resistance so that lower eddy current losses follow in magnetic cores made therefrom.

It has been discovered that chromium may be introduced in amounts of up to 5%, and preferably from 0.3 to 4%, into relatively easily worked silicon steel alloys of up to 5% silicon, and preferably 2.5% to 3.5 silicon, which chromium containing alloys are as readily and easily worked as the silicon steel Without the chromium, with development of a high electrical resistivity while exhibiting excellent magnetic properties. More importantly the addition of large amounts of chromium allows satisfactory cube grain growth to take place so that the siliconachromium-iron alloy sheets may be produced with over 70% [001] grain texture.

The object of the invention is to provide cube-on-face grain oriented magnetic sheets of iron-silicon-chromium alloys.

A further object of the invention is to provide magnetic sheets of a thickness of 8 mils and. greater comprising essentially an alloy of iron, silicon and from 0.1 to 5% chromium, with at least 70% of the grains having the (100) [001] orientation, the alloy sheets being characterized by high electrical resistivity.

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

The invention accordingly comprises the magnetic product possessing the features, properties and the relations of constituents which are exemplified in the follow- Patented Aug. 22, 1967 ing detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the drawings, in which:

FIGURE 1 is a view in side elevation of a stack of sheets illustrating how they are prepared for heat treatment;

FIG. 2 is a view in section of a furnace that could be utilized for heat treating a stack of sheets such as shown in FIG. 1;

FIG. 3 is a view in section taken along the lines III-III of FIG. 2;

FIG. 4 is a graph plotting electrical resistivity of additions of several elements to 3% silicon-iron alloy;

FIG. 5 is a graph plotting the effect of additions of several elements on the rate of growth of cube grains in 3% silicon-iron sheet;

FIG. 6 is a view of a punching in the form of a sector for a large dynamoelectric machine; and

FIG. 7 is a plan view of an E-punching.

Generally, the invention is directed to cold rolled and annealed magnetic sheets of an alloy composed essentially of a ferrous base alloy of from about 2% to 5% silicon, from 0.1% to 5% chromium, up to 0.4% manganese, small critical amounts of oxygen and sulfur, a carbon content of below 0.01%, balance iron except for incidental impurities, the magnetic sheet having a high proportion by volume, at least 70% by volume, of secondarily recrystallized grains with their crystal lattices having a cube face parallel within to the sheet surface and with four cube edges parallel within about to the sheet edge or rolling direction, while four other cube edges are perpendicular within about 15 to a direction transverse to the sheet edge or rolling direction, in Miller Indices these cube grains have the (100) [001] orientation. The alloy may contain small amounts of up to about 0.2% of molybdenum and up to 1% of nickel as permissible additions. The sheets are characterized by a high electrical resistivity which increases with the chro mium content. The alloy sheets will normally be prepared by unidirectional rolling. The sheets may be of any thickness, ranging for instance from 0.5 mil to 25 mils and greater. In the range of from 8 to 25 mils the magnetic sheets have great utility in transformers, motors and generators because they exhibit high permeability in two directions at right angles to each other.

One process for producing the sheets as set forth in US. application Ser. No. 428,813, includes (a) subjecting the alloy to at least two stages of cold rolling with at least one intermediate anneal at temperatures of from 975 C. to 1200 C., and preferably 1050 C. to 1200 C., in dry hydrogen or vacuum for a sufficient time to produce grain growth so that the average grain diameter is at least half the thickness of the sheet, and (b) a final anneal at a temperature of at least 1100" C. which results in first producing oriented cube grain nuclei in the sheet and then causing the nuclei to grow through the sheet material by a secondary recrystallization or exaggerated grain growth which produces a magnetic sheet with large grains having cube-on-face orientation such that the crystal lattice cube faces are closely parallel to the sheet surface and cube edges are closely oriented 'both to the direction of rolling and transverse thereto so that it exhibits a high permeability in both [100] directions which lie in the plane of the sheet, that is, parallel to and per pendicular to the rolling direction. It has been discovered that heavy gauge magnetic sheet, that is, about 8 mils to about 25 mils and heavier, can be produced in which from 80% to 100% by volume of the grains are so oriented that they have a (100) plane lying within 10 of the plane of the sheet, while from 60% to 85% and more of the cube-on-face grains are so oriented that they have a [100] direction or edge within 10 to 15 of the rolling direction or edge of the sheet.

In order to give a clear concept of the effectiveness of the process to be described hereinafter for orienting ironsilicon alloys and iron-silicon base alloys containing from 0.1% to 1% nickel, from 0.1% to 2% molybdenum, from 0.05% to 0.4%, and preferably 0.1% to 0.2%, manganese and from 0.1% to 5.0% chromium, or any two or more thereof, a list of representative alloys that have been treated successfully by the process will be described in detail. Specific illustrations of the silicon-chromium-iron alloy sheets of this invention also are given. The processes and results obtained will be given as the description proceeds in order that the effectiveness of the process and the merts of the magnetic sheets produced thereby may be clearly understood.

Alloys containing from 2.0% to about 4.0% by weight of silicon, 0.1% to about 5.0% by weight chromium and the remainder iron have also been cold rolled and annealed successfully into sheets with the high volume of double orientation grain texture. Good magnetic sheet resulted from ferrous alloys having 3% silicon and 2% chromium. It has been found that outstanding results are secured from alloys having from 2.0% to 4.0% by Weight silicon and from 2.0% to 0.1% by weight of chromium and from 0.05% to less than 0.4% by weight of man ganese and the remainder iron produce magnetic sheets that have exhibited a high volume of double orientation grain texture by the present processes. Preferred alloy compositions which are produced by the processes with excellent doubly oriented magnetic sheet resulting comprised 2.50% to 3.5% by weight silicon, from 0.3% to 4% chromium, up to 0.4% manganese, and the remainder iron.

The alloys of chromium with up to 0.2% molybdenum in particular have the advantage that they can replace silicon on an equal weight basis so as to produce an alloy lower in silicon but with magnetic and electrical resistivity characteristics as good as with a correspondingly higher silicon content, along with an improved grain texture and good workability.

In alloys containing both molybdenum and chromium, which have corresponding functions in producing the final product, the respective alloying elements may be varied from 0.1% to the maximum amounts indicated.

Manganese is a highly desirable constituent in any of the ternary and quaternary silicon-chmmiumdron base alloys, and it is generally preferable that manganese be present in all the alloys to enable the alloys to be worked better.

In processing alloys comprising from 2% to 5% silicon, up to 1% of nickel, not over 0.4% manganese, and not over about 0.2% molybdenum and from 0.1% to 5% chromium, the remainder being iron, it was found that the annealing step between cold rolling steps could be varied within the predetermined limits as will be described. Good success was obtained when more than one step of cold rolling was employed and the intermediate annealing was carried on for from /2 hour to 5 hours and longer in a range from 975 C. to 1200 C. to effect primary recrystallization and primary grain growth, the longer times at the lower temperatures, until the average grain diameter is at least half the thickness of the sheet.

During the final anneal the sheets should have an oxygen content of 0.005% or less for sheets of about 12 mils or less, and 0.003% or less when the sheets are 15 mils in thickness or above. For sheets of 5 mils and less in thickness the oxygen content at final anneal may be slightly more than 0.005%. For cube grain growth to take place, it is necessary that the sulfur content of the sheets at the time the secondary recrystallization is taking place should be less than 0.001% and preferably in the range of from 0.0006% to 0.0002%, with a minimum of at least 0.00003% sulfur present until secondary recrystallization is nearly complete. The ingots of the silicon-chromiumiron alloy initially may have somewhat more than 0.001 sulfur, for example 0.010%, but during the subsequent cold rolling steps the intermediate anneals will purify the metal so as to reduce the sulfur to much lower value. Also during the initial phases of the final anneal when primary recrystallization takes place, the sheet at final gauge will evolve sulfur such that when secondary recrystallization occurs the sulfur in the alloy is less than 0.001% and above at least about 0.00003 In the thinner gauges, for instance, 0.1 mil to 3 mils, the sulfur content in the sheets may be somewhat above 0.001% when the sheets enter the final annealing particularly if the sheets are being strip annealed, since sulfur is evolved quite readily in a hydrogen atmosphere. The intermediate anneal employs high temperatures, much higher than are usual in this art for such anneals, so as to help eliminate excess sulfur and also to provide that sulfur which tends to accumulate at grain boundaries as sulfides, such as manganese sulfide, is caused to dissolve throughout the grains of the iron-silicon alloy. The presence of larger grains after the intermediate anneal is one indication of such purification.

When silicon-chromium-iron alloy sheets are (1) cold rolled two or more times and intermediately annealed at least the final time between cold rolling steps at a temperature of about 975 C. to 1200" C. in dry hydrogen of a dew point of 30 C. or less and (2) subjected to a critical final heat treatment in excess of 1 hours, for example 2 to 16 hours, at 1100 C. to 1350" C. in an atmosphere of dry hydrogen having a dew point of from --40 C. to 60 C. or lower or in a vacuum of at least about millimeters of mercury so that surface oxides do not form or even disappear, the cube-on-face grain growth with double orientation is very satisfactory. In making the final anneal the use of all nickel or ironnickel alloy sheets containing over 20% by weight nickel disposed between the silicon-iron sheets or to enclose the magnetic material is helpful but not necessary.

From a large number of alloys subjected to the orienting process, the following nine examples have been selected. Some of them are disclosed herein for the first time as suitable for making oriented magnetic sheet. The compositions of the alloys are numbered so that they can be identified in the tables of test results which follow. The first five alloys have been suggested and treated by others using other processes. The alloys and their compositions, except for incidental impurities, are as follows:

TABLE I (1) 2.5% silicon, remainder iron.

(2) 3.0% silicon, remainder iron.

(3) 3.5% silicon, remainder iron.

(4) 4.0% silicon, remainder iron.

(5) 3% silicon, 0.3% manganese, remainder iron.

(6) 3% silicon, 0.2% nickel, remainder iron.

(7) 3% silicon, 0.5% molybdenum, remainder iron.

(8) 3% silicon, 0.3% nickel, 0.3% molybdenum, re-

mainder iron.

(9) 3% silicon, 0.3% chromium, remainder iron.

The foregoing alloys were prepared from electrolytic iron which is quite pure and silicon of a commercial grade having low aluminum content. Generally, this silicon is about 98% pure. Alloys 5 to 9, inclusive, were prepared from the same kind of iron and silicon and a relatively pure grade of manganese, nickel, molybdenum and chromium tas specified in each instance.

The melting of each alloy was carried on in a vacuum furnace having a magnesia crucible. After the metals to be alloyed were fed into the furnace, it was evacuated to a pressure of 0.1 micron of mercury. The charge was heated inductively in this particular instance. When the silicon began to melt, in order to prevent excessive evaporation of the metals, helium gas was admitted to the furnace and the alloying process continued. Helium was employed because it is inert. Argon and other inert gases may also be employed.

After melting was completed, each alloy was poured into a stainless steel slab mold to produce an ingot of ap- 6 proximately 5 pounds. This is the typical size of ingots employed for carrying on such investigations.

After each ingot was prepared in the manner described hereinbefore, a chemical analysis was made. It is not considered necessary to give an analysis of all the ingots. It is thought that it will sufiice to give the analysis of a 3% silicon-iron ingot. The analysis of the number 2 silicon-iron alloy after casting, in addition to iron, was as follows:

producing cube-on-face double orientation. The processes for treating the alloys will be identified hereinafter by the indicated letters to facilitate the making of comparisons. Processes A and B are prior art processes, while processes C and D represent the present invention.

PROCESS A In this process the ingot of each alloy is hot rolled at a temperature of about 1000 C. to a slab about 0.100 inch in thickness. The slap is then preferably treated to remove surface oxides by any of the well-known procedures. The removal of oxides immediately after the hot reduction is not vital; however, it is desirable to remove them before the final cold reduction.

The most commonly practiced process for removing the oxides is pickling in an acid. Sulfuric and hydrochloric acids are commonly employed in pickling processes. After the slab has been pickled to remove the oxides, it is drastically cold rolled in one step to reduce it to a sheet about 12 mils in thickness.

The final heat treatment which it applied to the coldrolled sheets and is common to this process and all of the other processes to be described hereinafter will 'be described in detail after method D has been disclosed.

PROCESS B In this process B the ingot is hot rolled at about 1000 C. to a slab of a thickness of about 0.100 inch. The slab is then pickled as described in process A to remove oxides.

The oxide-free slab is cold rolled to reduce it from a thickness of about 0.100 inch to about 0.070 inch. This is about a 30% reduction in thickness. A reduction of 30% is about the minimum that should be employed and about is the maximum. After this 30% cold-rolling deformation, the slab is annealed for about 2 hours at a temperature of about 800 C. in dry hydrogen at a dew point of -30 C. After the annealing step, the slab is again cold rolled to reduce it from a thickness of about 0.070 inch to about 0.030 inch. After the second deforma tion, the slab is again annealed for about 2 hours at about 800 C. in dry hydrogen at a dew point of 30 C. After both of these 800 C. intermediate anneals, the

grains had an average diameter for below half the sheet thickness, usually about .002 inch or less. Following the second anneal, the slab is cold rolled, reducing it from about 0.030 inch to about 0.012 inch. By this three-step cold-rolling process a sheet 12 mils in thickness is produced. This process is similar to that employed in producing single or Goss orientation of the grains in the direction of rolling. (See US. Goss Patent 1,965,559.) This final cold-rolled sheet is heat treated or annealed as disclosed after method D.

PROCESS O slab may then be pickled to remove oxides. While it is usual to remove oxides after hot rolling, it is not always required, however, the oxides should be removed before the last cold reduction.

The next step is a cold-rolling operation to reduce the slab from about 0.100 inch to about 0.070 inch which is about a 30% reduction. After this cold reduction, the slab is annealed for about 2 hours at about 1000 C. in dry hydrogen having a dew point of -30 C. Then follows another cold-rolling step reducing the slab from about 0.070 inch to about 0.030 inch, which is about a 60% reduction. The slab is again annealed for about 2 hours at about 1000 C. in dry hydrogen at a dew point of at least 30 C. While an anneal of 2 hours between cold-rolling steps is preferable, benefits may be obtained with anneals ranging from /2 hour to 5 hours at temperatures from 975 C. to 1200 C. in an atmosphere of dry hydrogen having a dew point of at least 30 C. such that primary recrystallization and the desired grain growth occur. The grain size after this intermediate anneal averages better than 0.015 inch. A final cold-rolling step is employed to reduce the slab from about 0.030 inch to a sheet which is about 0.012 inch in thickness, which is a 60% reduction. Other percentage cold reductions may be employed depending on conditions, but the reductions should be at least 30% and not more than 80%. A final heat treatment described after method D is to be given to the 12 mil sheet.

PROCESS D This process is a modification of process C. In practicing the process, the ingot is first hot rolled at 1000 C. to reduce it to a slab about 0.100 inch in thickness. It may be then or later pickled to remove oxides.

After or before the oxides have been removed from the slab, it is cold rolled to reduce it from about 0.100 inch to about 0.070 inch in thickness. After this cold deformation the slab is annealed for about 1 hour at about 600 C. to 800 C. in wet hydrogen having a dew point of about 25 C. followed by an anneal for 1 hour at about 975 C. to 1200 C. in dry hydrogen at a dew point of -30 C. The slab is again cold rolled, reducing it from about 0.070 inch to about 0.030 inch in thickness. The sheet is given a second anneal comprising A hour to 2 hours at 600 C. to 950 C. in wet hydrogen (dew point up to 25 C.) followed by /2 hour to 5 hours at 975 C. to 1200 C. in dry hydrogen at a dew point of at least -30 C. The average grain size after this intermediate anneal is more than 0.015 inch, usually being 0.025 inch or more. After the second anneal, the slab is cold rolled to reduce it from about 0.030 inch in thickness to a sheet about 0.012 inch in thickness.

In process D the wet hydrogen portion of the intermediate anneals may be carried out at temperatures of from 600 C. to 950 C., and the wet hydrogen may have a dew point from 0 C. to 25 C. The wet hydrogen anneal may be applied for from about A hour to 2 hours. It is quite important that no wet decarburization be apapplied after the sheet is cold rolled to final gauge, lest a thick oxide film be present which will prevent cube-onface grain growth.

FINAL ANNEAL Each of the processes A to D inclusive described hereinbefore requires one more step after the final cold rolling, which is a critical final heat treatment. In the final heat treatment after each method A, B, C and D, the 12- mil sheets whose surfaces are quite clean and free from continuous oxide films, are heat treated for from 2 hours to 16 hours at about 1200 C. in dry hydrogen having a dew point as low as 60 C. to effect a complete secondary cube-on-face grain growth such that the entire sheet comprises substantially cube-on-face grains. The dry hydrogen should result in the sheets being bright at the end of the anneal. A vacuum of an absolute pressure of from to 10- mm. of Hg was employed with equally gOOd results. The sheet surfaces are bright after this anneal. While the preferred temperature for the final anneal is about 1200 C., it may be varied from 1100 C. to 1350 C. The time to produce substantially complete secondary recrystallization may be an hour or somewhat less at the highest temperatures, and more than 20 hours at the lower temperatures. The hydrogen should have a dew point of at least 40 C. In each case the sulfur content of the sheets during the final anneal was in the range of from 0.00003 to 0.0006%, while the oxygen content was below 0.003%.

Prior to the final heat treatment step of each process, the alloy sheets are coated with an aluminum oxide powder to separate the sheets from each other during final anneal. The aluminum oxide should be fine and quite dry to avoid the introduction of moisture into the furnace. The aluminum oxide selected should be relatively pure. Preliminary firing of the aluminum oxide at 1200 C. is desirable. In practicing the process, a -mesh size powder was employed satisfactorily.

The alloy sheets 10 coated with a layer of the aluminum oxide powder 11 are then placed between nickel or nickel alloy sheets 12, as shown in FIG. 1. The size of the stack shown in FIG. 1 is only limited by the number of alloy sheets 10 that can be treated in the furnace at one time. While in the modification of the invention illustrated a nickel or nickel alloy sheet is placed between all adjacent magnetic sheets, this is not required to practice the processes. In some cases there may be nickel or nickel alloy sheets placed around the stack of magnetic sheets.

The stacked sheets of FIG. 1 may be treated in any suitable type of furnace which is provided with the necessary equipment for controlling the atmospheric conditions. The furnace shown generally at 13 in FIGS. 2 and 3 is an electric furnace provided with resistor elements 14 disposed in spaced rows. The furnace chamber comprises a nickel alloy tube 15 which is disposed between the rows of resistor units 14.

In order to facilitate the loading of the stack of sheets shown in FIG. 1 into the tube in the furnace, a boat 16 is provided to hold the stack of sheets. While not necessary, it may be desirable to make the boat from a nickel alloy. While the annealing of a stack of fiat sheets is shown in FIG. 1, it will be understood that a coil may be employed wherein the turns are separated by a layer of dry alumina or other non-reactive sheet separator.

The annealing furnace shown in FIG. 1 is essentially of laboratory or pilot plant type. Commercial annealing furnaces, such as bell-type furnaces, will be employed for the final annealing of large coils or stacks which latter may be fiat sheets or punchings of the final cold rolled sheet.

As pointed out hereinbefore, proper heat treatment of the magnetic sheets requires careful control of atmospheric conditions. The magnetic sheet should be heat treated in an atmosphere of dry hydrogen of a dew point of at least -40 C. or a vacuum of at least 10*. of Hg, and relatively free from oxygen or oxidizing components so that the sheets anneal bright. In this particular furnace, the dry hydrogen is supplied through a pipe 17 having its inner end curved back on itself to direct the hydrogen delivered to the furnace over the stack of sheets carried by the boat 16. The tube 17 through which the hydrogen is delivered extends the length of the furnace so that as the hydrogen flows through the tube, it is preheated be fore it comes in contact with the stack of sheets.

In the heat treatment procedure, when the boat 16, loaded with the stack of magnetic sheets shown in FIG. 1, is properly positioned in the furnace, hydrogen is delivered through the pipe 17 completely flushing out the nickel alloy tube 15. After the tube 15 has been properly cleared of deleterious gases and filled with dry hydrogen, it will be sealed, though a slight flow of hydrogen is usually passed through the furnace during the entire anneal.

As set forth in the specification, the heat treatment will be carried on preferably for about 16 hours at a temperature of 1200 C., in an atmosphere of dry hydrogen of a dew point of at least 40 C. After the heat treatment is completed, the seal will be removed from the tube 15 to give access to the boat 16. In order to avoid any harmful effects from the combination of gases in the atmosphere with the hydrogen when the seal is removed, the tube is flushed with a suitable inert gas such as helium. While it is preferable to continue the heat treatment for 16 hours at 1200 C. in dry hydrogen having a dew point of at least -40 C., satisfactory results have been obtained with heat treatments of from 2 hours to 16 hours in the dry hydrogen.

When the heat treatment of the magnetic sheet is completed, the boat with its contents may be removed. The layer of aluminum oxide 11 applied to the magnetic sheets 10 in the stacking operation prevents any adhesion between the silicon-iron sheets or sticking of the nickel alloy sheets 12 to the magnetic sheets 10. The stack of fully treated magnetic sheets may be separated readily.

The final heat treatment described hereinbefore was given to sheets processed by methods A, B, C and D and brought about secondary recrystallization or an exaggerated growth of cube-on-face grains. The final product is a doubly oriented magnetic sheet with secondary grains which have diameters that exceed twice the thickness of the sheet.

A study of the nine sheets given in the Table I hereinbefore following the processing was made using domain pattern methods. The orientation given in the Table II which follows comprises, first, the percentage of the total sheet area or volume which is comprised of grains having (100) plane orientation within 10 of the plane of the sheet, and second, the percentage of these cube-onface grains which have a [100] direction within 15 of the rolling direction.

In all of these examples in Table I the oxygen was of the order of 0.002% and less while the sulfur was on the order of 0.001% when the sheets were put in the furnace during the final anneal. Sulfur was evolved during the initial phases of this final anneal so that the sulfur in the sheets during secondary recrystallization was 0.001% and less. All the sheets were mirror bright at the end of the anneal.

Samples 2, 4, 5 and 8 were not processed by Process D and consequently no tests were made thereon.

' TABLE II.ORIENTATIONS OF SILICON-IRON SHEETS Cube-on-Faco Grains Having Grains Having [100] Direction Alloy Process (100) Planes Within Within 15 to 10 to Sheet Plane, Rolling Direction,

Percent Percent A 80-100 5 A 80-100 5 A 80-100 5 A 80-100 5 A 80-100 5 A 80-100 10 A 80-100 5 A 80-100 5 A 80-100 5 B 80100 5 B 80-100 B 80-100 5 r B 80-100 5 B 80-100 60 B 80-100 35 13 80-100 5 B 80-100 5 B 80-100 5 0 80-100 60 C 80-100 65 C 80-100 75 0 80-100 50 C 80-100 80 C 80-100 50 0 80-100 70 C 80-100 70 C 80-100 85 D 80-100 75 D 80-100 00 D 80-100 75 D 230-100 70 D 80-100 75 Table II shows that a high percentage of secondray cube-on-face grains grows in each specific alloy sheet following each rolling procedure or process. This is evidenced by the high percentage of grains oriented with the (100) plane in the plane of the sheet. The percentage of grains whose [100] direction is within 15 of the rolling direction resulting from processes A and B was extremely low, except for alloy 5 treated by the B process which comprises 60%. The processes C and D disclosed herein wherein there was large grain growth during the intermediate anneal, were quite effective in bringing about a high percentage of grains having the [100] direction within 15 of the rolling direction.

As evident from Table II, in the nine alloys treated by process C and the five alloys by process D, there was an unexpected and highly desirable improvement in the grain edge orientation of each alloy sheet. The high percentage of grains having a [100] direction within 15 of the rolling direction is all the more important since the sheets processed were 12 mils thick. It will be noted from the Table II given hereinbefore that alloys 5 and 9 have a high percentage of grains oriented with a [100] direction in the rolling direction. This indicates that the composition of the alloy and. the intermediate annealing temperature of about 975 C. to 1200 C. with dry hydrogen employed between the successive steps of cold rolling cooperate to effect the required high degree of orientation of both cube faces: and cube edges.

Further examples of the process are as follows, Examples I and II being hydrogen and vacuum final annealing for the straight silicon-iron alloys.

Example I A commercial heat of silicon steel of the following composition was hot rolled to 0.080-inch sheets, silicon 3.15%, manganese 0.11%, sulfur 0.019% (in hot rolled sheet), carbon about 0.015%, oxygen about 0.0015%, balance iron except for slight amounts of impurities. The hot rolled plate was annealed for one hour at 1050 C. in dry hydrogen (55 C. dew point), and then cold rolled to 0.030 inch. The cold rolled sheet was annealed for 20 minutes in wet hydrogen (10 C. dew point) in order to decarburize the silicon steel, and then annealed for one hour at 1100 C. in dry hydrogen (55 C. dew point). The average grain size was 0.7 millimeter. The sulfur content also was greatly reduced to about 0.002%. The annealed sheet was again cold rolled to final gauge of 0.012 inch on highly polished rolls. The surface was clean and free from oxides. The sheet was annealed for 16 hours at 1200 C. in dry hydrogen (55 C. dew point).

The sheet was mirror bright after the final anneal and the sulfur content was below 0.001%. Analysis of the grain texture indicated over 95% of the sheet volume comprised cube-on-face secondary grains with the cube faces within 5 of the plane of the sheet surface, and about of the cube grains had their cube edges within 10 of the rolling direction. The electrical resistivity of the sheet was about 48 microhm-cm. Magnetic tests of the sheet at 10 oersteds gave an induction (B) of 18,600 gauss. The magnetic properties in both the direction of rolling and transverse thereto were substantially equal.

Example II for two hours at 1100? C. in dry hydrogen having a dew point of below 40 C. The average grain diameter was in excess of 0.04 inch after this anneal. The sheet was then finally cold rolled from 0.040 inch to 0.012 inch.

Strips cut from the cold rolled 0.012 inch sheet were annealed at 1200 C. in a vacuum of between and 10- mm. of Hg for 16 to 20 hours. The strip oxygen content was approximately 0.002%. The strips were stacked between nickel chromium plates using alumina powder for separation. The surfaces of the strips were bright after the final anneal and had a sulfur content of less than .0006% after this final anneal. Over 90% of the volume of the strips was composed of secondary grains with a cube face within 5 of the sheet surface, and over 70% of the cube-on-face grains had their cube edges within of the rolling direction. The electrical resistivity was about 46 microhm-cm.

Example III Sheets of an alloy composed of 3% silicon, 1% chromium and the balance iron with very small amounts of impurities, were prepared by initially vacuum melting electrolytic iron and appropriate amounts of highly purified chromium and silicon, and pouring an ingot therefrom under helium atmosphere. The ingot was hot rolled into a sheet of 0.100 inch thickness which was then pickled and cold rolled in two stages, first, to a thickness of 0.025 inch (75% reduction), followed by an intermediate anneal for 24 hours at 1200 C. in hydrogen gas of a dew point of -50 C., the grains in the intermediate annealed sheet having an average diameter of more than twice the sheet thickness, and, second, cold rolled to final gauge of a thickness of 0.011 inch (56% reduction). The sheet had an oxygen content of below 0.003% and the sulfur content was below 0.0007%. The 0.0l1-inch thick sheet was finally annealed at 1200 C. for 24 hours effecting complete secondary recrystallization in an atmosphere of 50 C. dew point hydrogen to which was added parts per million of hydrogen sulfide to insure that in the sheet there was an adequate sulfur content of above 0.00003% while secondary recrystallization was taking place.

The fully annealed sheet was mirror bright. More than 90% of the grain volume of the sheet had a (100) [001] texture whose (100) faces were within 10 of the sheet face. The B values of the sheet as determined by magnetic tests were 18,100 gauss. On 60-cycle tests, the watt losses at an induction of 15,000 gauss Were 0.57 watt per pound of magnetic material. The 10 cycle losses at an induction of 17,000 gauss were 0.82 watt per pound. These are remarkably low losses. The electrical resistivity of the sheet was 52 microhm-cm. The coercive force H, was 0.068 oersted.

Example IV An alloy was prepared, cold rolled and annealed by the technique of Example 111, having a composition of 3% silicon, 2% chromium and 0.1% manganese, balance iron. The finally annealed 0.0l1-inch thick sheet had over 90% by volume of (100) [001] secondary recrystallized grains. Magnetic tests indicated B values of 17,600 gauss. The electrical resistivity of the sheet was 58 microh-m-cm.

Example V An ally ingot having a composition of 3.2% silicon, 4% chromium, and 0.1% manganese, balance iron and traces of impurities was melted as in Example 111, hot rolled to a sheet of 0.08 inch thickness, cold rolled to 0.012 inch thickness, intermediate annealed at 1200 C. in 50 C. dew point hydrogen for 24 hours, and cold rolled to 0.006 inch thickness. The alloy was easily worked and cold rolled. The oxygen and sulfur were both less than 0.001%. After a final anneal carried out as in Example 111, over 90% by volume of the finally annealed sheet comprised (100) [001] secondary recrystallized grains having crystal lattice faces within 10 of the sheet surface, and 77% of these grains had cube edges Within 10 of the rolling direction. The electrical resistivity of the sheet was 73 microhm-cm.

Example VI Four ingots of 3% silicon iron were prepared by vacuum melting electrolytic iron and high purity additions, ingot 10 containing no other elements than iron and silicon, ingot 11 containing 0.1% chromium, ingot 12 containing 1% chromium and ingot 13 containing 2% chromium. All the ingots were hot rolled to sheets 0.060 inch thick, pickled, annealed 1 hour at 1050 C. in dry hydrogen, and cold rolled to a thickness of 0.016 inch. The sheets were annealed at 1150 C. in hydrogen of a dew point below 50 C. for 15 minutes to develop a primary recrystallization structure and the grains were measured. Samples of the sheets were etched in a H O -H PO solution of a thickness of 0.012 inch.

The 0.012 inch sheets with oxygen below 0.003% and sulfur above 0.-00003% were annealed at 1200 C. for up to 70 minutes in hydrogen of a dew point of below 50 C. to obtain cube grain growth and the rate of cube grain growth was determined by measuring the resulting grains and comparing their sizes with the earlier primary grains and the points plotted in FIG. 5. The resistivity of the sheets was as follows:

In a manner similar to Example VI, other alloy sheets were prepared with silicon, nickel, copper, manganese, and molybdenum additions in the basic 3% silicon-iron alloy and both the electrical resistivity and cube grain growth rate were determined for varying amounts of each addition and plotted in FIGURES 4 and 5.

The curves establish that chromium gives an increase in resistivity slightly less than that of an equal weight of silicon, however the growth of cube texture grains is reduced far less by the additions of chromium than for an equal weight of silicon.

Thus, the addition of 2% chromium to a 3% siliconiron alloy reduces the rate of cube grain growth from about 0.47 mm. per minute to 0.25 mm. per minute, whereas 2% silicon additions to make a total of 5% silicon reduce the growth rate to about 0.04 mm. per minute. The 2% chormium-3% silicon-iron alloy grows cube grains at almost 6 times the rate of 5% silicon-iron alloy. Coupled with the far greater ease of cold working the 2% chromium-3% silicon alloys as compared to the 5% silicon alloy, there are great and unexpected advantages in the cube-on-face sheets of this invention;

It should be noted that both copper and molybdenum greatly reduce the cube grain growth rates and are generally to be avoided if reasonable rates of grain growth are to be secured.

Manganese is shown to have a moderate effect in increasing electrical resistivity, and is closely similar to silicon in its effect in reducing the rate of cube grain growth up to about 0.5%. Due to its beneficial function in improving the workability of the alloys, manganese when added in small amount of up to about 0.2%, and not exceeding 0.4%, is a desirable alloy component.

Nickel up to 1% does not apprecicably change the electrical resistivity of the silicon-iron alloys, but it is closely similar to silicon in its effect on reducing the rate of cube grain growth.

In the range of from about 2.5 to 3.5% silicon, from about 0.3 to 4% chromium, from 0.05 to 0.4% manganese, balance iron sheet magnetic materials having outstanding properties by reason of a high electrical resistivity and over 70% by volume of (100) [001] grain texture are obtained.

The important function of the high temperature intermediate anneal is to enable a much closer or sharper alignment of four of the cube edges of the "crystal lattices of the cube-on-face grains to the direction of rolling. A simple low temperature stress relief anneal which results in a fine grain texture will not result in a close orientation of the edges of the cube grains to the direction of rolling. Thus, Example 11, after a simple intermediate stress relief anneal at 750 C. to 800 C; of the cold rolled sheet, followed by another cold rolling to final gauge and then a final anneal, will result in cube-on-face grain growth, but with less than 50% of the grains having cube edges within 10 of the rolling direction.

The alloy compositions disclosed hereinbefore may be varied within the limits indicated to meet different requirements. The thickness of the magnetic sheet produced may be less than 8 mils. The final anneal he-at treatment must be carried out with great care and precision as indicated above.

It will be understood that the high temperature intermediate anneal of this invention resulting in coarse grain growth need be applied only once to the sheet between any two cold rolling steps if three or more cold rolling steps are employed. This high temperature intermediate anneal preferably precedes the final cold rolling. It may be employed for all the intermediate anneals, or the sheets may be intermediately annealed only once at the high temperatures, while the other anneals may be the usual low temperature 'anneals at temperatures up to 900 C. for brief periods of time, including wet hydrogen decarburization anneals.

The silicon-chromium-iron alloy cube-'on-face oriented magnetic sheets of this invention may be employed to great advantage for v-arious magnetic core structures for transformers, motors and generators for example. Illustrative of the utility of such laminated core structures are cores produced by combining in stacked cores L- punchings, E-punchings and other punchings configurations having portions at right angles to each other, such that one of the two directions of easiest magnetization is in the direction of each of these perpendicular portions whereby during use of the cores magnetic flux is most efficiently carried in the punchings. Also more efiicient motor and generator cores, which are usually of curved configuration, may be prepared from the double oriented sheets of this invention.

Referring to FIGURE 6 of the drawing there is illustrated core sectors as produced from the sheets of this invention suitable for use in producing stators of large motors and generators. The sheet 20 having the grain texture in the (100) 1] orientation, has easiest and most efiicient directions of magnetization in the directions 22, parallel to the sheet edge or rolling direction, and 24 perpendicular thereto. A plurality of lamination sectors 26 are punched from sheet 20 in the orientation shown so that the teeth 28 which are to carry high flux densities in service are either parallel or closely parallel to direction 22 when electrical windings are disposed in slots 30, while the lamination back 32 is generally closely parallel to the direction 24 so that magnetic flux passing between the teeth has a path of low reluctance and high efiiciency.

FIGURE 7 shows an E-punching 50, which may be employed for transformers, relays and other electrical apparatus, as produced from the (100) [001] texture sheets of this invention. The punching 50 comprises a back 52, and legs 54, 56 and 58 projecting at right angles thereto. The back 50 is cut from the sheet so that it is parallel to direction 60 which may be parallel to the sheet edge and therefore parallel to a [100] direction of easiest magnetization while legs 54, 56 and 58 are parallel to the other [100] direction.

For magnetic cores, sheets or strips of the alloy after final anneal will usually be provided with a thin electrically insulating coating of an inorganic material such, for example, as a magnesium oxide, mica, magnesium phosphate, or a glass. When the strip is wound into a core or the insulated sheets are punched into laminations and stacked, the insulation reduces any eddy current losses. The wound or stacked cores can be bonded with a resin, a glass or an inorganic cement so that they can be cut and the faces ground to produce a core joint of low magnetic reluctance. The thickness of the insulation and/or bonding material should be small, for example, 0.1 to 1 mil, to provide for a high space factor.

Since certain changes in carrying out the above process, and certain modifications in the article which embodies the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. Magnetic sheet material comprising a cold worked and annealed alloy of from 2% to 5% silicon, from 0.1% to 5% chromium, up to 0.4% manganese, and the balance iron except for incidental impurities, the sheet comprising over 50% of its volume of [001] grain texture, such grains having (100) planes within about 10 of the sheet surface, the magnetic sheet characterized by a high electrical resistivity exceeding that of the alloy sheet without the chromium being present.

2. The magnetic sheet of claim 1, wherein over 70% of the grain volume comprises secondarily recrystallized grains having the (100) [001] texture.

3. The magnetic sheet of claim 2, wherein the alloy includes up to 0.2% molybdenum and up to 1% nickel.

4. The magnetic sheet material of claim 1 wherein the sheet thickness is from 8 to 25 mils.

5. A magnetic sheet material comprising a cold worked and annealed alloy consisting essentially of from 2% to 3.5% of silicon, from 0.3 to 4% chromium, up to 0.4% manganese, and the balance iron except for small amounts of impurities, the sheet being of a thickness of from about 0.5 to 25 mils in thickness, the sheet comprising at least 70% by volume of secondary recrystallized grains having (100) [001] orientation within an angle of about 10 of the cube faces to the sheet surface, the magnetic sheet characterized by a high electrical resistivity exceeding that of the alloy sheet without the chromium being present and excellent magnetic properties in the direction of the sheet edge and a direction perpendicular thereto.

6. The magnetic sheet of claim 5, wherein the alloy comprises about 3% silicon and from 1 to 4% chromium.

7. A magnetic core comprising a plurality of superposed laminations of the magnetic sheet material of claim 1 and an electrically insulating material disposed between the laminations.

References Cited UNITED STATES PATENTS 2,209,687 7/1940 Crafts 148-111 2,867,557 1/1959 Crede et al. 148-111 2,867,559 1/1959 May 148-111 2,940,881 6/ 1960 Hollomon 148-111 2,940,882 6/1960 Hibbard et al. 148-111 2,992,951 7/1961 Aspden 148-111 2,992,952 7/1961 Assmus et al. 148-111 3,008,857 11/1961 Mobius 148-111 3,034,935 5/1962 Walter et al 148-111 3,147,157 9/1964 Grenoble 148-111 DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

N. F. MARKVA, Assistant Examiner.

Claims

1. MAGNETIC SHEET MATERIAL COMPRISING A COLD WORKED AND ANNEALED ALLOY OF FROM 2% TO 5% SILICON, FROM 0.1% TO 5% CHROMIUM, UP TO 0.4% MANGANESE, AND THE BALANCE IRON EXCEPT FOR INCIDENTAL IMPURTIES, THE SHEET COMPRISING OVER 50% OF ITS VOLUME OF (100 (001) GRAIN TEXTURE, SUCH GRAINS HAVING (100) PLANES WITHIN ABOUT 10* OF THE SHEET SURFACE, THE MAGNETIC SHEET CHARACTERIZED BY A HIGH ELECTRICAL RESISTIVITY EXCEEDING THAT OF THE ALLOY SHEET WITHOUT THE CHROMIUM BEING PRESENT.