US2939810A - Method for heat treating cube-on-edge silicon steel - Google Patents

Description

United States Patent M C METHOD FOR HEAT TREATING CUBE-ON-EDGE SILICON STEEL Howard C. Fiedler and Robert H. Pry, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Dec. '31, 1956, Ser. No. 631,476 3 Claims. Cl. 148-111 This invention relates to polycrystalline magnetically so 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 theircrystal space lattices are arranged in a substantially identical rela-' tionship to the plane of the sheet and to a single direction in the plane of the sheet, and more particularly, to a particular composition of such materials which is particularly amenable to treatment by an improved process for formingsheet material having this desired orientation.

The sheet materials to which our invention is directed are usually referred to in the art as electrical silicon steels or, more properly, silicon irons and are conventionally composed principally of iron alloyed with about 1.5 to 4 percent and preferably about 2.5 to 3.5 percent silicon and relatively minor amounts of various impurities such as sulfur, manganese, phosphorus and having very low carbon content as finished material. Such alloys 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 forniing the individualcrystals or grains assume in such materials. In these materials the smallest prism posessing the. full symmetry of the crystal is termed the unit'eell 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 or body-centered unit cubes comprising these materials each have a high degree of magnetic anistropy with respect to the crystallographic planes and directions of the unit cube, and hence, each grain or crystal comprising a plurality of such unit cell-s exhibits a similar. 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 perpendicularto a plane passed through diagonally oppositeparallel unit cube edges and their least easiest direction of magnetization perpendicular to a plane passed through a pair of diagonally opposite atoms -in afirstz'unit cube face, the central atom and a single atom in the. unit cube face whiohis 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 'besfound in Structure of Metals, C. S. Barrett,

and the (111) plane and the [L11] direction.

be fabricated by unidirectional rolling and heat treatment acterized by having 50 percent or more. of its component grains oriented so thatfour of the cube edges of the unit cells of said grains are substantially parallel to the plane of the sheet or strip and to the direction in which it was rolled and a (110) crystallographic plane sub stantially parallel to the plane of the sheet. be seen that these so-orientedgrains have a direction of; easiest magnetization in the plane of the sheet in arolling direction and the next easiest direction of magnetization in the plane of the sheet in the transverse to rolling direc-. tion. 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 and in the rolling direction may approach the maximum attained in single crystals in the direction.

In actual steel mill practice, these materials are prepared by casting ingots from alloys containing from about 2.5 to 4.0% and preferably from 2.5 to 3.5 percent by weight silicon, less than 0.035 percent carbon, about 0.02 to 0.03 percent sulfur, and less than 0.15 percent man: ganese. These ingots are conventionally hot worked into a strip or sheet-like configuration, usually less than 0.150 inch in thickness referred to as hot rolled band. This material is usually in an incompletely recrystallized form and may be annealed to effect complete recrystalli zation if deemed desirable but it is usually not necessary.

The hot rolled band is then cold rolled with appropriate annealing treatment to the finished sheet or stripthiokness usually involving at least a 50 percent reduction in thickness and given a final or texture producing annealing treatment. As presently practiced, this final anneal is accomplished in two steps. First, a short time normalizing anneal is accomplished at about 800 C. for about five minutes in a wet hydrogen or wet cracked gas? atmosphere. This anneal serves at least two purposes. It decarburizes the material or stated otherwise, reduces the carbon content of the material to a value of lessthan 0.030 percent by weight, and additionally causes the worked metal structure to recrystallize into a fine grain microstructure. This is usually referred to as a primary recrystallization. Because of the relatively low temperature and short time involved in this anneal, it is possible to employ a continuous annealing technique wherein the sheet or strip of metal is fed through a controlled atmosphere furnace at a rate such that any given portion of the strip is raised to the required temperature for the necessary period' of time. Such continuous annealing techniques are Widely employed in the metallurgical arts and are usually more economical than batch anneals. The decarburized strips or sheets are then cooled and coated with a refractory material and, depending on their size and configuration, either coiled or stacked and placed in an enclosed box which is provided with an atmosphere Patented June 7, 1960 It will thus able period of time order to accomplish the two processes previously stated and to produce acceptably strong textures. This has required the employment of a batch type anneal. The time required for abatch anneal of this usually requires from one to two days since in order to" accomplish the anneal in the most economical fashion, large amounts of metal are annealed in each batch; a

After annealing, the sheet or strip material must then be flattened to remove warping which usually occurs during the final anneal. This is usually accomplished by heating the strip or sheet and applying tension thereto.

It would be desirable to reduce the amount of time necessary in the final annealing treatment. Accordingly, it isa principal object of our invention to provide an iron" base silicon alloy which may be fabricated in a shorter length of time than in previously known alloys to produce sheet material having the desired degree of (51 [001] grain orientation.

- It is afurther object of our invention to provide a substantially continuous process whereby cold rolled strip or' sheet-like bodies of silicon-iron may be subjected to controlled atmosphere heat treatment to produce a desired degree-of (110) [001] grain orientation therein.

Other and specifically different objects of our invention' will become apparent from the detailed disclosure which follows.

Strip and sheet grain oriented silicon-iron alloys have been used for a number of years as transformer core materials, electric motor and generator laminations and in other electrical and electronic applications where the high electromagnetic properties in the rolling direction of the sheet or strip may be advantageously employed. For most applications the highest degree of grain orientation obtainable is desirable. Usually materials having more than about 70- percent of their crystal structure oriented in the (110) [001] texture are considered to have a strong texture.

It has been found that strong textures in conventional silicon-iron alloys aredependent upon the presence of sulfur, probably present as a dispersion of manganese sulfide during the second recrystallization phase of the final annealing treatment during which time the texture is developed. This has been demonstrated by the fact that strong textures cannot be developed in high purity silicon-iron alloys prepared from vacuum melted substantially pure iron, silicon and carbon. However, when such alloys are made with controlled amounts of man- :ganese and sulfur introduced as impurities, the material can be fabricated to produce strong textures. Unfortunately', it is necessary to remove substantially all the sulfurin order to attain optimum magnetic properties since the presence of sulfur in such materials exerts a deleterious effect upon these properties. We have discovered that the development of the desired strong texture may be accomplished in these silicon-iron alloys without the addition of sulfur and concomitantly that the necessity for removing sulfur by means of a long time duration batch anneal requiring coating the sheet or strip metal with a refractory material to prevent sticking to- 'gether during the anneal and the necessity for a flattening operation after the anneal is eliminated. Further, the rate at which the material of our invention may be heat treated to produce a low carbon silicon-iron alloy having more than 70 percent of its crystal structure oriented in the desired texture is sufiiciently rapid that a continuous final annealing process may be advantageously and economically adopted.

Briefly stated, our invention utilizes a relatively small amount of titanium, probably present as a dispersion of titanium carbide, effectively to control the secondary recrystallization of the silicon-iron alloys without sulfur being present. In essence, our titanium containing alloys may be cast, hot reduced to band and the hot-rolled band then cold reduced to sheet or strip of the final thickness with appropriate intermediate heat treatment. This cold worked material is then subjected to a heat treatment of a short time duration during which time a strong (110) [001] texture is developed, preferably by means of a continuous anneal. The strip or sheet material is then heat treated at a slightly higher temperature for a short time duration, again preferably in a continuous annealing apparatus whereby the carbon is substantially removed and the finished material is permitted to cool. Since this final annealing treatment may be accomplished with thersheet orstrip material under sufficient tension to pre-- vent warping, a subsequent flattening operation may be avoided, and since the strip or sheet is not in contact with other sheets or stripsduring the annealing, no refractory coating is necessary.

More specifically, a number of heats or different alloys were made of which the following listed percentages of compositions are representative.

Table 1 Alloy Si 0 T1 S O N In. the preceding table, alloys 1-4 were prepared by melting appropriate amounts of. substantially pure commercial electrolytic iron containing less than 0.01 per.- cent manganese, silicon as low aluminum 98 percent ferrosilicon substantially pure.- sponge titanium, and carbon as an iron-carbon alloy made. from electrolytic iron and graphite in an electric induction furnace in an air atmosphere. Alloys. 5-7 were prepared from the same basic materials but melted and castin a vacuum furnace. The air melted heats were cast into ingots measuring about. 1." x 5 /2" x 13" and the vacuum. melted heats cast into ingots measuring about 1" x 3" x 6". The compositions listed in. Table I. are the results of chemical. analyses of: the alloys as cast. It should be noted that no sulfur or manganese were added to these alloys. The trace of sulfur found in. each alloy is probably due to impurities inthe raw materials. or from the refractory of the furnace crucibles.

These ingots; were: each heated to about. 1000 C.. and rolled without reheating to strip or hoterolled band? mils thick. This rolled material was then annealed at 900' C. fora half hour in dry (dewpoint about 60 F.) hydrogen to elfect complete recrystallization. However, this anneal may be. omitted if desired, and protective atmospheres other than hydrogen may be used. The annealed. bands were thenv cold rolled to 25 mils thicknms and annealedat' 860 C. for 2 minutes in dry hydrogen, thencold'roll'ed. to 12 mils; thickness. It should be noted that this intermediate. annealing temperature is not critical butshould be maintained between about 850 and 950 C. for optimum results. Further, since the function of' this anneal is to cause recrystallization of the worked metal and not primarily to reduce the excess carbon content, or carbon which is notcombined with titanium as TiC, protective atmospheres other than hydrogen or a vacuum anneal may equally well be used. Specimens of this cold rolled strip or sheet material were then. subjected to a first annealing treatment comprising heating for between 15 to 20 minutes at about 1000 C. in dry hydrogen. After this heat treatment, it was found that these materials had. recrystallized so that about 80% of their grains had the desired [001] texture as determined by conventional torque tests, and that substantially no change had occurred in their chemical compositions.

These specimens were then subjected to a second heatsion of titanium carbide.

treatment in dry hydrogen for varying periodsof' time at 100050. and "1050 C. in order to remove substantially all the'carbon. "As an example, the following table illustrates the elfect of time and temperature during this anneal upon the carbon content in sheet specimens of alloy 1. If it is assumed that all'the titanium in this particular alloy is present as titanium carbide (TiC), then on a weight. percent basis, the alloy contains about 0.10% TiC and any excess carbon is probably present as iron carbide. It will be seenthat the carbon content is lowered to about the assumed valuefor TiC during the processing prior to and including the first portion of the final heat; treatment and that prolonged heating at 1000 C. is; not elfective' to further lower thecarbon content. However, heating other specimens for much shorter periods of time reduces the carbon content to a fairly stable value of about 0.007% carbon.

Table II Annealing Temperature Time It should be pointed out that if heavier gauge finished sheet or strip material is desired, for example, 25 mil thick material, the same general procedure may be followed except the hot rolled band should be annealed in a protective atmosphere, the annealed band cold reduced to the desired final thickness, e.g. 25 mils, and this cold worked metal then subjected to heat treatment in a protective atmosphere to cause the desired recrystallized texture to develop. The recrystallized material may then be heat treated in a hydrogen atmosphere to cause the decomposition of the titanium carbide phase with the attendant removal of the carbon.

It will be apparent from the foregoing that by utilizing an alloy containing from about 2.5 to 4% silicon, with sutficient titanium and carbon to combine at least 0.01% of the alloy as the compound titanium carbide, less than about 0.010% sulfur and the balance substantially all iron, substantial improvements may be accomplished according to our invention in the processing steps necessary to produce the electrical grade grain oriented sheet and strip material. It will be appreciated that the hydrogen atmosphere annealing employed will be effective to remove excess carbon present as iron carbide.

As a probable theory to explain the recrystallization behavior and the carbon removal which occurs during the final heat treatments of the cold rolled material, the carbon in these alloys is probably present as a fine disper- Since the cold worked alloys can be caused to recrystallize at temperatures between about 750 C. and l000 (3., and since the titanium carbide phase does' not substantially go into solution in the silicon-iron matrix at temperatures below about 1050 C., primary recrystallization of the cold worked material is accomplished at its temperature is raised during the initial stage of the first part of the annealing treatment. As the first part of the final anneal progresses secondary recrystallization phenomenon takes place wherein the titanium carbide particles probably inhibit the growth of grains of orientations other. than (110) [001]. It should be appreciated that this recrystallization process is a time-temperature dependent reaction, i.e., that while recrystallization may be accomplished at temperatures below 1000 C., the rate is slower at lower temperatures. Furthermore, if the heat treatment is accomplished at temperatures above the solution temperature f 'the'titanium carbide phase, strong textures will not develop. j ,,After the secondary recrystallization is substantially completed, the temperature may then be raised tojor above 1050" 0., probably causing the titanium carbide phase to go into solution in the matrix whereupon the carbon atoms may freely migrate to the surface and readily dissipate in the final decarburizing step.

In view of the relatively short times required for recrystallization and decarburization of the relatively thin sheet or strip materials, a continuous, annealing treatment may be readily substituted for the time con suming batch annealing treatment necessary with con ventional analogous material.

While for purposes of a complete disclosure, certain specific compositions, melting practices, rolling schedales, and heat treatments have been particularly set forth, it will be readily apparent to those skilled in the art that variations therefrom may be employed without departing from the scope of our invention. For example, other low sulfur content irons may be employed in place of electrolytic iron or the molten alloy may be sub jected to a desulfurizing treatment whereby for example, the sulfur may be removed in a slagging operation. Additionally, the hot rolling may be accomplished with periodic reheating if deemed desirable and the final thickness of the hot rolled band may vary substantially from that disclosed. For example, it is conventional to terminate the hot working at thicknesses as great as 100 mils or greater under some circumstances. Additionally, the cold reduction of the hot rolled material may be accomplished in one or more than the two rolling sequences specifically disclosed, however, the material at the final cold worked stage should have been exposed to at least 40% cold reduction prior to the final annealing treatments. Yet, further, while the final annealing treatment may be accomplished in two separate furnaces, it will be apparent that a single furnace having two temperature zones may be employed in a continuous anneal. In this case, the cold Worked metal enters the lower temperature zone of the furnace, is recrystallized therein and progresses into the higher temperature zone where decarburization is accomplished.

For the foregoing reasons We do not intend our invention to be limited in any manner except as defined in the appended claims.

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

1. A method for producing a polycrystalline sheetlike body having a majority of the constituent grains oriented in the (110) [001] direction, comprising preparing an alloy melt consisting essentially of from about 2.5 to 4.0 weight percent silicon, less than about 0.010 weight percent sulfur, and not more than about 0.035 weight percent carbon, adding up to 0.10 weight percent titanium to said alloy melt forming from 0.01 to 0.10 weight percent titanium carbide dispersion therein, casting said titanium carbide-containing alloy and hot reducing said casting to form an elongated sheet-like body less than 150 mils in thickness, cold rolling said body to elfect at least a 40 percent reduction in thickness, heat treating said cold-worked body in a protective atmosphere at a temperature from 750 to 1000 C. to cause the desired [001] recrystallized texture to develop, and subsequently effecting a decarburizing heat treatment on: said body at a temperature not less than about 1050 C. so that decomposition of the titanium carbide phase is effected with removal of the carbon from the metal body.

2. A method as defined in claim 1 in which said hot reduced elongated sheet-like body is subjected to a heat treatment to cause the complete recrystallization of the body prior to cold reduction.

3. A method for producing a polycrystalline sheetlike body having a majority of the constituent grains *7 onientedint the; (110)- 10011 direction, comprisingppre paring an alloy melt consisting essentially of from about 2.-5 ;to 4.0- weight percentsilicon, less than about 0010 Weight percent, sulfur, and not more thanabout 0.035. weight percent carbon, adding upto 0.10 weight percent titanium to, said-al1oy meltforming from 0.01 to 0.10 weight percent titanium carbide dispersion therein, casting said titanium:carbide-containing alloy and hot reducing said casting to form an elongated sheet-like body less'than 150 milsin thickness, cold rolling said body to effect at least a 4.0 percent reduction in thickness, heat treating said cold-worked body in a protective atmosphere at a'ternperature from 850 to 950 C. to cause the desired (110) [001] recrystallized texture to develop, and subsequently effecting a decarburizing heat 8 treatment-on said body at a temperature not less than about 105.0? so that decomjaositibnofithe titanium carbide-phase is efiected with removal of. the carbon from themetalsbcrdyttj l nerer negfcittn in the me or this patent '5 UNITED- STATES PATENTS 1,553,488 Thu aud ;...Sept. 15, 1925 2,158,065 Cole et a1. May 16, 1939 2,173,312 Rohn e't-al. Sept. 19 1939 2,209,686 Crafts; July 30, 1940 2,378,321 P-akkala June 12, 1945 2,826,520 Rickett Mar. 11, 1953 2,867,558 1959 May 5...; Jan. 6,

Claims (1)

1. A METHOD FOR PRODUCING A POLYCRYSTALLINE SHEETLIKE BODY HAVING A MAJORITY OF THE CONSTITUENT GRAINS ORIENTED IN THE (110) (001) DIRECTION, COMPRISING PREPARING AN ALLOY MELT CONSISTING ESSENTIALLY OF FROM ABOUT 2.5 TO 4.0 WEIGHT PERCENT SILICON, LESS THAN ABOUT 0.010 WEIGHT PERCENT SULFUR, AND NOT MORE THAN ABOUT 0.035 WEIGHT PERCENT CARBON, ADDING UP TO 0.10 WEIGHT PERCENT TITANIUM TO SAID ALLOY MELT FORMING FROM 0.01 TO 0.10 WEIGHT PERCENT TITANIUM CARBIDE DISPERSION THEREIN, CASTING SAID TITANIUM CARBIDE-CONTAINING ALLOY AND HOT REDUCING SAID CASTING TO FORM AN ELONGATED SHEET-LIKE BODY LESS THAN 150 MILS IN THICKNESS, COLD ROLLING SAID BODY TO EFFECT AT LEAST A 40 PERCENT REDUCTION IN THICKNESS, HEAT TREATING SAID COLD-WORKED BODY IN A PROTECTIVE ATMOSPHERE AT A TEMPERATURE FROM 750 TO 1000*C. TO CAUSE THE DESIRED (110) (001) RECRYSTALLIZED TEXTURE TO DEVELOP, AND SUBSEQUENTLY EFFECTING A DECARBURIZING HEAT TREATMENT ON SAID BODY AT A TEMPERATURE NOT LESS THAN ABOUT 1050*C. SO THAT DECOMPOSITION OF THE TITANIUM CARBIDE PHASE IS EFFECTED WITH REMOVAL OF THE CARBON FROM THE METAL BODY.