US3069299A - Process for producing magnetic material - Google Patents

Description

1962 H. c. FIEDLER PROCESS FOR PRODUCING MAGNETIC MATERIAL Filed Sept. 16, 1959 Fig.

I000 las'a run! c I 900 l-l/VAL AN/VE'AL/NG- TEMPERA Fig.2.

I IIOO C l l l .900 .950 I000 I030 FINA L A NIVEAL lA/G- 7' 5MP! RA TUAE Inventor-1 Howard C.

United States Patent Ofilice 3,959,299 Patented Dec. 18, 1962 This invention relates to the fabrication of polycrystalline, magnetically soft, rolled sheet metal composed principally of an alloy of iron and silicon having a high percentage of the grains comprising the material oriented such that their crystal space lattices are arranged in a substantially identical relationship to the plane of the sheet and to a single direction in the plane of the sheet, and more particularly to an improved process for forming sheet material having this desired orientation.

This application is a continuation-impart of application Serial No. 693,043, entitled Magnetic Material, filed October 29, 1957, now abandoned, and assigned to the same assignee as the present invention.

The sheet materials to which this invention is directed are usually referred to in the art as electrical silicon steels or, more properly, silicon-iron, and are conventionally composed principally of iron alloyed with about 2.5 to 4.0 percent, preferably 2.5 to 3.5 percent silicon and relatively minor amounts of various impurities, such as sulfur, manganese, phosphorous and a very low carbon content as finished material. Such alloy-s crystallize in the body-centered cubic crystallographic system at room temperature. As is well known, this refers to the symmetrical distribution or arrangement which the atoms forming the individual crystals or grains assume in such materials. In these materials, the smallest prism possessing the full symmetry of the crystal is termed the unit cell and is cubic in form. This unit cube is composed of nine atoms, each arranged at the corners of the unit.

cube with the remaining atom positioned at the geometric center of the cube. Each unit cell in a given grain or crystal in these materials is substantially identical in shape and orientation with every other unit cell comprising the grain.

The unit cells which are body-centered unit cubes comprising these materials each have a high degree of magnetic anisotropy with respect to the crystallographic planes and directions of the unit cube, and hence, each grain or crystal comprising a plurality of such unit cells exhibits a similar magnetic anisotropy. More particularly, crystals of the silicon-iron alloys to which this invention is directed are known to have their direction of easiest magnetization parallel to the unit cube edges, their next easiest direction of magnetization perpendicular to a plane passed through diagonally-opposite parallel unit cube edges and their least easiest direction of magnetization perpendicular to a plane passed through a pair of diagonally-opposite atoms in a first unit cube face, the central atom and a single atom at the unit face which is parallel to the first face. As is Well known, these crystallographic planes and directions are conventionally identified in terms of Miller indices, a more complete description of which may be found in Structure of Metals, C. S. Barrett, McGraw-Hill Company, New York, N.Y., 2nd edition, 1952, pages 1-25, and are conventionally referred to as, respectively, the (100) plane and the corresponding [100] direction, the (110) plane and the [110] direction, and the (111) plane and the [111] direction.

It has been found that certain of the silicon-iron alloys may be fabricated by unidirectional rolling and heat treatment to form sheet or strip material composed of a plurality of crystals or grains, a majority of which have their atoms arranged so that their crystallographic planes have a similar or substantially identical orientation to the plane of the sheet or strip in a single direction in said plane. This material is usually referred to as Oriented or grain-oriented silicon-iron sheet or strip and is characterized by having 50 percent or more of its component grains oriented so that four of the cube edges of the unit cells of said grains are substantially parallel to the plane of the sheet or the strip and to the direction in which it was rolled and a (110) crystallographic plane substantially parallel to the sheet. It will thus be seen that these so-oriented grains have a direction of easiest magnetization in the plane of the sheet in the rolling direction and the next easiest direction of magnetization in the plane of the sheet in the transverse rolling direction. This is conventionally referred to as a cube-on-edge orientation or the (110) [001] texture. In these polycrystalline sheet and strip materials, it is desirable to have as high a degree of grain orientation as is attainable in order that the magnetic properties in the plane of the sheet in the rolling direction may approach the maximum attained in a single crystal in the direction.

In actual steel practice, these materials are prepared by casting large, thick section ingots weighing up to about 4 tons each'from alloys containing from about 2.5 to 4.0 percent by weight silicon, less than 0.035 percent carbon, less than 0.05 percent sulfur, and less than 0.15 percent manganese. By thick section, it will be understood that such ingots usually have a minimum transverse crosssectional dimension of about two feet. Such ingots are conventionally hot-worked into a strip or sheet-like configuration, usually less than 150 mils in thickness, commonly referred to as hot-rolled band. This band material is usually in an incompletely recrystallized form and may be annealed to effect complete recrystallization, if desired, but this is usually not done in conventional commercial practice.

The hot-rolled band is then cold-rolled with appropriate intermediate heat treatment to the finished sheet or strip thickness, usually involving at least a 40 percent 7 reduction in thickness, and given a final or texture producing annealing treatment accompanied by a decarburizing treatment.

It has been the conventional mill practice to strip the ingot molds from the ingots as soon as practicable while the ingots are quite hot and to immediately place them in a soaking pit and heat them to a minimum temperature of about 1300" C. to 1400 C., at which temperature they arehot-rolled,

It i a principal object of this invention to provide silicon-iron alloy castings having a fine dispersion of second phase particles which assist development of crystalline orientation by controlling grain growth during the final anneal.

It is an additional object of this invention to provide a process for treating silicon-iron ingots to acquire maximum [001] crystalline orientation.

Other and specifically different objects of this invention will become apparent to those skilled in the art from the detailed disclosure which follows.

In the drawings,

FIG. 1 is a graph showing the percent cube-on-edge crystal orientation developed from silicon-iron cast in graphite and sand molds, as a function of the final annealing temperature; and

FIG. 2 is a graph similar to that of FIG. 1 in which silicon-iron bodies cooled at rates of 50 C. and C. were used.

Briefly stated, the present invention is predicated upon the discovery that there is a previously unsuspected relationship between the rate of cooling of a silicon-iron alloy ingot containing selected amounts of manganese and sulfur from temperatures at which the manganese and sulfur are in solution, for example, 1300 to 1400 C. or higher and the degree of oriented crystal texture which can be ultimately developed. By cooling the silicon-iron ingots from temperatures on the order of 1400 C. to about 800 C. at a rate of not less than about 50 C. per minute and preferably faster, for example, at about 130 C. per minute, a fine dispersion of manganese sulfide is produced which retards normal grain growth during the final anneal and thereby enables a high degree of crystal orientation to be produced.

Generally, the metal can either be cooled directly from the liquid condition, as when originally poured into an ingot mold, or can be cooled from a temperature in excess of that at which the manganese and sulfur are normally in solution, this latter temperature normally being on the order of 1300-1400 C. If the silicon-iron is cooled either from or above these temperatures at a sufiicient rate, the manganese sulfide dispersion is properly distributed throughout the ingot and no further treatment need be carried out. This material is perfectly acceptable for subsequent rolling to produce a cube-ou-edge crystal orientation. Of course, if the ingot is so large as to make it difficult to attain the required cooling rate, then it can be rolled rapidly to reduce the cross-section and thereby increase the cooling rate. It has been found that if the material is cooled at a rate of 50 C. per minute or faster, preferably on the order of about 130 C. per minute, then a fine manganese sulfide dispersion is obtained.

To clearly illustrate the effect of temperature on the degree of cube-on-edge orientation which can be obtained from a material, two 50-pound ingots were cast from the same heat of metal into a graphite mold in one case and into a sand mold in the other case. The purpose in using the different types of molds was to test the effect of different cooling rates which were obtained due to the different degrees of thermal conductivity of the two types of molds. The composition of the metal in this instance was 3.27 percent silicon, 0.026 sulfur, 0.057 manganese, 0.004 carbon, 0.009 oxygen, 0.002 nitrogen, and the remainder substantially all iron.

The ingot molds were 2% inches by 5 inches in crosssection and one-inch thick slabs were cut from the ingots, heated to 1000 C., rolled without reheating to 80 mil band, pickled, sand-blasted and heat-treated in the band stage for 5 minutes at 900 C. The band material was then cold-rolled to 25 mils in thickness, heat-treated at 860 C. for 1 to 5 minutes and cold-rolled to 12 to 13 mils.

The faster is the cooling rate either from or above the solution temperature of the manganese sulfide, the smaller are the manganese sulfide particles and the more effective they are in preventing normal grain growth in the final gauge strip. For example, the material which was cast in sand and processed to final gauge had a grain size of about 0.038 millimeter when heated for minutes at 950 C., whereas the material which had been cast in graphite and processed to final gauge had a grain size of about 0.030. To ascertain the cooling rates of the metals cast in graphite and sand molds, separate one-inch test slabs were heated to above the solution temperature for the manganese and the sulfur (l3001400 C.) and then cooled at rates of 50 C. per minute and 130 C. per minute. Final gauge strip from the piece cooled at 130 C. per minute had an average grain size of about 0.020 millimeter, while a similar strip from the material cooled at 50 C. per minute had an average grain size of approximately 0.028 millimeter. It will be readily noticed that the strip from material cooled at 50 C. per minute had a grain size generally the same as that of the strip from material cast in the graphite mold. From this data it may be concluded that the material cast in the graphite had a cooling rate on the order of 50 C. per minute,

while the material cast in sand had an appreciably lower cooling rate.

The effect of the cooling rates can clearly be seen by referring to the graphs shown in FIGS, 1 and 2 of the drawings where the amount of cube-on-edge texture obtained after two hours at the temperature indicated is shown. The degree of cube texture developed in the material cast in graphite can be seen by referring to the curve designated by the numeral 10. In this case, textures as high as about percent were obtained when the material wa heat-treated within the range of from about 925 to 975 C. On the other hand, the material processed from the ingot which was cast in the sand, and therefore had a cooling rate of less than 50 C. per minute, had a maximum texture of only on the order of 40 percent when heated at from 925 to 975 C.

Referring to the two curves shown in FIG. 2 of the drawings, the curve 15 represents strip from that body which was cooled at a rate of 130 C. per minute, while the numeral 16 indicates strip from that material which was cooled at about 50 C. per minute. It will be noted that the textures obtained from the material cooled at 50 C. per minute correspond substantially with that indicated by curve 10 in FIG. 1. Here, once again, the maximum texture obtained was on the order of 80 percent and the texture obtained dropped off rapidly as higher final annealing temperatures were used. This feature of decreasing orientation with increasing annealing temperatures is an important one since it permits the use of only comparatively low heating rates in the texturedeveloping anneal. The percent cube-on-edge texture developed is shown in Table I as a function of the heating rate during the final anneal.

Table I Percent TextureI-Ieating Rate Slab (gooling Rate One difficulty involved is the fact that the texture takes a substantially longer time to develop at 950 C. than it does at some higher temperature, for example, 1050 to 1100 C. In this connection, the material which was cooled at 130 C. per minute developed textures approaching percent and textures in excess of 80 percent, even when the annealing temperature was raised to 1100 C. Thus, if the cooling rate is kept at a substantially high level to precipitate a fine, grain-boundary pinning second phase, i.e., the manganese sulfide or other inclusion, then high orientation can be readily and easily developed through use of the higher temperatures. It has been observed that annealing times on the order of one-half hour are needed to develop adequate texture at temperatures of about 950 0, whereas complete texture development can occur in as little time as 5 minutes when the annealing temperature is raised to 1050 to 1100 C. or possibly slightly higher. It is thus obvious that while the cooling rate of 50 C. per minute will permit the attainment of good cubc-on-edge textures, the higher cooling rates, for example, the C. per minute, permit even further advantages to be realized.

All of the textures were measured by the conventional torque magnetometer test described later in the specification.

Additional examples illustrating the invention are set forth in the following representative examples. In these examples, a number of heats or alloys of silicon-iron having comparable compositions as shown in Table II were prepared by melting commercial electrolytic iron containing less than 0.01 percent manganese by spectrographic analysis, with appropriate amounts of silicon as com- 'r'nrcial, low aluminum, 98 percent 'ferrosilicon, sulfur as iron sulfide, carbon as an iron-carbon alloy made from electrolytic iron and graphite, and titanium in the form of titanium sponge. With regard to the titanium addition, it will be appreciated that it is desirable that the sulfur not be present in these alloys as-cast as iron sulfide in order to obtain optimum rolling characteristics. In usual commercial practice, manganese is used for this purpose, forming manganese sulfide; however, titanium may equally well be used, forming titanium sulfide. Obviously, manganese and titanium may be used or, in fact, other addition elements known to form stable sulfides may also be used.

All of the representative heats weighed 50 pounds and were made in an induction furnace with magnesia cruc1- bles. The alloys were poured at temperatures between 16001650 C. into various sized molds made of different materials. For example, heats 1, 4 and 5 were cast into cast iron molds having a regular mold cavity having a rectangular transverse cross-section with a minimum transverse width of 3%" and a minimum thickness of 1%" and a length of 18". Heats 2 and 3 were cast into graphite molds having a rectangular transverse crosssection with a minimum of transverse width of 6%.", a minimum thickness of 3 A and a length of 9". Heats 6 and 7 were cast into cast iron molds having a regular mold cavity having a square transverse cross-section with a minimum transverse width and thickness of 3% and a length of 14". The ingots having the 1%" thickness will cool substantially faster than the ingots of 3%" thickness.

The ingots were cast into the molds, permitted to cool, removed from the molds and subjected to the following fabrication procedure.

Ingots from heats 1, 4 and 5 were machined to form 1%" thick by 3%" wide slabs, heated to 1000 C. 1n a dry hydrogen (dewpoint about 60 F.) atmosphere and hot-rolled to 80 mil thick hand. These hot-rolled bands were then annealed at 900 C. for /2 hour at that ternperature in an atmosphere of dry hydrogen. It should be noted that any conventional protective atmosphere or a vacuum may be used in place of hydrogen at this point. After cooling, the annealed bands were cold-rolled to mil thickness strip. These strips were then subjected to an intermediate anneal in a dry hydrogen atmosphere at about 860 C. Again, other protective atmospheres or vacuum may be used. The strips were in the heated zone for about 3%. minutes and at the 860 C. temperature for about one minute. After cooling, the grain sizes of these anneal strips were determined and found to range from an average measured diameter of 0.017 to 0.025 millimeter. The annealed strips were then cold-rolled to 12 mil thick strip and decarburized by heating for 5 minutes .at 800 C. in wet hydrogen. The decarburized strips were then placed in'a metal retort in a furnace and dry hydrogen (90 dew point) was flowed through the retort as temperature was increased at a rate of 100 C. per hour to 1200 0, held at that temperature for 8 hours and cooled at about 100 C. per hour to 600 C., then furnace-cooled to about 300 C., at which point the retort was removed from the furnace.

The ingots from heats 2 and 3 were processed as follows: A slice of about 1" thick by 6%" wide by 9 long was cut from each ingot, the resulting slabs heated to 1000 C. and hot-rolled to mil thick hot-rolled bands. The bands were annealed in the same manner as the bands from heats 1, 4 and 5 and cold-rolled to 25 mil thick strip. One portion of the strip from heat 2 was then annealed in dry hydrogen under the same conditions set forth for the intermediate anneal given strips from heats 1, 4 and S and another portion from heat 2 and all of strip from heat 3 were annealed under the same conditions except the temperature was raised to 950 C. The intermediate grain size of the 860 C. anneal strip from heat 2 had an average measured grain size of 0.007 millimeter, while the 950 C. annealed portion of this same material had a grain size of 0.021 millimeter. The intermediate grain size of the strip from heat 3 was 0.025 millimeter. Both strips from heat 2 and the strip from heat 3 were then cold-reduced to 12 mils thickness and given the decarburization and final annealing treatment given the strips from heats 1, 4 and 5.

The ingots from heats 6 and 7 were heated to 1000 C. in dry hydrogen and forged .to a rectangular transverse cross-section of 1%" by 4". A portion of each forged bar was then reheated to 1.000 C. in dry hydrogen and rolled to 80 mil thicknesshot-rolled hand. These bands were annealed as set forth previously and cold-reduced to 25 mil intermediate thickness strip and heated in the same manner in similar strips from ingots l, 4 and 5 at 860 C. The intermediate grain size from the strip from heat 6 was found to be 0.027 millimeter and from the strip from heat 7 to be 0.016 millimeter.

Conventional torque magnetometer test specimens, oneinch diameter disks, were prepared from each finished strip and were rotated, as is well known, in a unidirectional magnetic field of 1000 oersteds. The degree of (110) [001] preferred orientation or texture expressed as percent texture was determined by torque measurements for each sample and is reproduced in Table III. This value is calculated from the torque values found for the specimens based on the value of 150,000 ergs/ cubic centimeter for percent texture as found in single crystals of analogous composition having the specified orientation. Additionally, the final carbon content and final sulfur content was determined for these materials.

Table III Intermediate Grain Percent Final Final Ingot Size (mm.) Texture Carbon Sulfur (Percent) (Percent) Under certain circumstances, heavier gauge finished sheet or strip-like material may be desired. For example, if 25 mil thick sheet having a high degree of orientation is desired, the previously disclosed hot-rolled 80 mil band should be annealed to effect complete recrystallization. This may be accomplished by annealing the band at about 900 C. for a period of time necessary to effect complete recrystallization. The recrystallized band may be then cold-reduced to the final thickness, e.g., 25 mils, without intermediate heat treatment, decarburized and subjected to the same final annealing treatment as set forth with respect to the 12 mil strips previously disclosed. As an example of the effectiveness of this treatment to produce strong textures, samples of the hot-rolled band from heat 1 were cold-rolled to the 25 mil thickness with and with- V Table IV Percent Percent; Texture Texture (Band not (Band Annealed) Annealed) It will be seen, therefore, that strong textures are developed in this heavier sheet or strip material only if the hot-rolled band is recrystallized before cold-working.

From the material set forth in detail in the present specification, it may be seen that the cooling rate of the as-cast structure through the temperature range where the sulfide first precipitates and through the temperature range immediately below that temperature range where growth of these particles may occur is critical to the degree of preferred grain orientation which may be achieved in the final strip material.

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

1. A process for forming a fine dispersion of sulfide inclusions in silicon-iron castings which are adapted to be formed into sheet-like bodies having a majority of their constituent grains oriented in the (110) [001] texture comprising cooling said casting from a temperature at which the sulfides are in solution to about 1000 C. at a rate of at least 130 C. per minute, reducing said casting by rolling to form a sheet-like body, and annealing said sheet-like body to develop said grain orientation therein.

2. A process for the fabrication of sheet-like bodies of polycrystalline silicon-iron alloy having a majority of their constituent grains oriented in the (110) [001] texture comprising the steps of casting an alloy consisting essentially of from about 2.5 to 4.0 percent silicon, less than about 0.04 percent carbon, from about 0.010 to 0.05 percent sulfur and the remainder substantially all iron, said sulfur being present as sulfide inclusions in the form of a fine dispersion in said casting at temperatures below about 1300 C., cooling the casting at a rate of at least 130 C. per minute from a temperature 8 at which the sulfides are in solution to about 1000" C., producing a metal sheet from said casting, and annealing said metal sheet to refine the same and to develop grain orientation therein.

3. A process for the fabrication of sheet-like bodies of polycrystalline silicon-iron alloy having more than a majority of their constituent grains oriented in the [001] texture comprising the steps of melting an alloy consisting essentially of from about 2.5 to 4.0 percent silicon, less than 0.035 percent carbon, from about 0.015 to 0.05 percent sulfur, and the remainder substantially all iron, casting said alloy into a mold to produce a slab-like ingot, said sulfur being present as sulfide inclusions in the form of a fine dispersion in said castings at temperatures below about 1400 C., cooling said ingot at a rate of at least C. per minute to a temperature between room temperature and less than 1000 C., removing said ingot from said mold, heating said ingot to about 1000 C., rolling said heated ingot to reduce said minimum transverse dimension to form an elongated sheet-like body less than mils in thickness, cold-rolling said elongated body to effect at least a 40 percent reduction in thickness and annealing said cold-worked body in a suitable reducing atmosphere to cause the desired recrystallized texture to develop and to purify the strip.

4. A process as defined in claim 3 in which said hotreduced elongated sheet-like body is subjected to a heat treatment to cause the complete recrystallization of the body prior to cold reduction.

5. A process as defined in claim 3 in which said hotreduced elongated sheet-like body is cold-rolled to effect at least a 40 percent reduction in thickness, heated in a protective atmosphere to cause the cold-Worked metal to recrystallize, and cold-rolled to eifect at least a 40 percent reduction in thickness prior to the decarburization and texture-development heat treatments.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,141 Morrill et al. Dec. 12, 1950 2,618,843 Goodsell Nov. 25, 1952 2,867,558 May Jan. 6, 1959 OTHER REFERENCES

Claims (1)

1. A PROCESS FOR FORMING A FINE DISPERSION OF SULFIDE INCLUSIONS IN SILICON-IRON CASTINGS WHICH ARE ADAPTED TO BE FORMED INTO SHEET-LIKE BODIES HAVING A MAJORITY OF THEIR CONSTITUENT GRAINS ORIENTED IN THE (110)(001) TEXTURE COMPRISING COOLING SAID CASTING FROM A TEMPERTURE AT WHICH THE SULFIDES ARE IN SOLUTION TO ABOUT 1000*C. AT A RATE OF AT LEAST 30*C. PER MINUTE, REDUCING SAID CASTING BY ROLLING FOFM A SHEET-LIKE BODY, AND ANNEALING

Images