US3446680A - Production of grain-oriented silicon steels - Google Patents

Description

May 27, 1969 3,446,680

PRODUCTION OF GRAIN-ORIENTED SILICON STEELS C. A. CLARK ET Filed June 20, 1968 IIOO SW M a mawmwm T w w wmy f SwU W Z CPu United States Patent US. Cl. 148-112 6 Claims ABSTRACT OF THE DISCLOSURE A method of producing Goss texture in a steel sheet containing from 3.5% to 6.5% silicon, from 2% to 12% nickel, the nickel being present in an amount sufiicient to render the steel rollable at 100 C. and 'being correlated with the silicon content, such that as the silicon is increased from 3.5% the amount of nickel is increased to 2%, from 0.02% to 0.06% carbon, and from 0.05% to 0.3% vanadium, the balance being iron except for impurities, which comprises effecting secondary recrystalliz-ation in the steel by heating it at a temperature below that of the transformation point of alpha iron to alpha plus gamma iron. The secondary recrystallization treatment also effects at least substantial decomposition of the precipitated vanadium carbides. Said treatment may be carried out in an atmosphere of wet hydrogen.

The present application is a continuation-in-part of our copending application Ser. No. 477,688 filed Aug. 6, 1965, and now abandoned.

The present invention relates to grain-oriented steel production and more particularly to a special heat treating process which, when applied to silicon steels of special composition, provides grain-oriented steels characterized by a highly satisfactory combination of magnetic and other properties.

As is Well known to those skilled in the art, grainoriented silicon steels have very valuable magnetic properties and are extensively used as laminations in transformers and other apparatus for this reason. In the production of these steels, an essential step is cold rolling (a term which includes warm rolling at, say 100 C.) to produce orientation of grains. It is commonly accepted that the optimum results are obtained when 0.080 inch hot rolled sheet is cold rolled to a thickness of 0.014 inch with intermediate annealing. In the cold rolling steps, the steel is deformed and in the annealing steps primary recrystallization t-akes place at temperatures over about 400 C. If the steel is heated to a high enough temperature and contains an inhibitor of the growth of primary grains, secondary recrystallization to give the so-called Goss texture occurs. In this texture anedge of the cube in the body-centered cubic structure of the steel is parallel to the direction of rolling and a simple diagonal plane is parallel to the plane of the sheet or strip.

The usual, if not the most common, inhibitor of primary grain growth heretofore employed, that is to say, the agent for facilitating the production of Goss texture, is manganese sulfide, and in practice, manganese and sulfur are invariably added to the steel to provide this sulfide. In practice, the secondary recrystallization is effected at a temperature of the order of about 1,2 O C. primarily because the optimum texture is produced at this temperature.

Although the manganese sulfide is required in order to produce an adequate amount of Goss texture, it is deleterious to the magnetic properties if it remains in the steel and most of the sulfur is, therefore, removed when it has served its purpose. This is often accomplished by coating 'the steel with magnesia before heating it to cause secondary recrystallization. During this heating, the manganese sulfide decomposes if the temperature is as high as about l,200 C. Substantially all the manganese goes into solution in the iron and the sulfur is removed by diffusion and combination at the surface with magnesia while it is serving its essential purpose in promoting secondary recrystallization. The fact that 1,200 C. is the order of temperature required to decompose manganese sulfide is a secondary reason for effecting the secondary recrystallization at this temperature.

Typically, the rolled steel 0.014 inch thick is decarburized by heat treatment for five minutes at 820 C. in wet hydrogen. The steel is then cooled to room temperature and coated with magnesia and finally the recrystallization to give the Goss texture is effected in dry hydrogen at 1,200 C. and in the course of it the sulfur content is reduced to a maximum of 0.003%.

The silicon in the steel increases the electrical resis'tivity and so reduces the eddy current 'losses and it also reduces the hysteresis loss. It is known that the optimum silicon content for minimum magnetostriction is about 6.3% but air melted steels heretofore commercially produced and containing more than about 3.5% silicon are so brittle that they cannot satisfactorily be cold rolled. It is possible to increase the silicon content to about 4.0% and still cold roll the steel if the melting is performed under vacuum but, of course, this increases the expense of manufacture. Thus, at the present time commercially produced, grain-oriented silicon steel normally contains about 3.2% silicon (with a tolerance of 0.2%), not more than about 0.003% carbon, not more than about 0.003% sulfur and from about 0.05% to 0.15% manganese. The "balance is iron, impurities being kept at low as possible.

Recently it has been found that it is possible to increase the silicon content of grain-oriented steels while still keeping the steel cold rollable by incorporating special amounts of nickel in the steels. This is described and claimed in US. Patent No. 3,238,073 which issued on Mar. 1, 1966. It has now been found that by further modification of both the composition of the steel and of the heat treatment, We can produce steel that, when magnetized, has magnetic properties superior to those of the standard silicon steels extensively used today. This involves the use of special amounts of vanadium in siliconnickel steels and the utilization of low secondary recrystallization temperatures.

It is an object of the present invention to provide a special heat treatment which, when applied to silicon steels of special composition, results in grain-oriented silicon steels of enhanced magnetic characteristics.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 depicts a relationship between the silicon and nickel contents of the steels;

FIGURE. 2 sets forth illustratively the iron-silicon phase diagram; and

FIGURE 3 is a graphical presentation of various curves which indicate various phase boundaries of steels containing certain silicon contents, the nickel content being varied.

In the steels in accordance with the present invention, the silicon content thereof is from 3.5 to 6.5% and there is present an amount of nickel within the range 3 of 2% to 12% sulficient to render the steel rollable at 100 C. The amount of nickel required depends on and is correlated with the silicon content, increasing as the silicon increases. The preferred correlation in which the nickel content must vary with the silicon content for this purpose is shown in FIG. 1 of the accompanying drawing, in which curve A shows the minimum content for this purpose. It will be seen, for instance, that at 4% silicon there should be preferably at least about 3.7% nickel, and :at 5% silicon at least 6.5% nickel. At 6% silicon, as much as 8% nickel is required for optimum rolla'bility at 100 C. It is desirable to add no more nickel than is required to render the steel rollable at 100 C. since an excess of nickel in comparison with silicon leads to loss of magnetic properties. We find that steels nominally containing 4% silicon give excellent properties and in these steels we prefer to inculde a nominal amount of 4% nickel. The tolerance in each of these percentages is 0.3% and, therefore, the preferred steels contain from 3.7% to 4.3% silicon and from 3.7% to 4.3% nickel.

An important factor in the choice of the composition of the steel is the existence of the so-called gamma loop in the iron-silicon phase diagram. This is shown at B in FIGURE 2 of the accompanying drawing. Within the loop an iron-silicon alloy is austenitic (i.e., in the gamma form) or in the hatched part of the loop, partly ferritic (i.e., in the alpha form) and partly austenitic. If Goss texture is to be produced, the steel must be wholly ferritic during the recrystallization. Now, as explained above, the temperature of secondary recrystallization of the nickel-free alloys is of the order of 1,200 C. or above and it will be seen that unless the steel contains 3% or more silicon, it will come within the loop at the secondary recrystallization temperature and this is indeed the reason why nearly all previous grain-oriented silicon steels have contained 3.2% silicon.

In contrast to silicon, nickel is an austenite former and the addition of nickel enlarges the gamma loop. The curve C of FIGURE 2 is approximately the loop in a steel containing nickel. When we found the approximate shape of the loop in various nickel-containing silicon steels, it appeared that at the necessary recrystallization temperature they would be at least partly austenitic and, therefore, it would be impossible to produce satisfactory Goss texture in them. It has been surprisingly found that it is possible to produce a satisfactory Goss texture in a nickel-containing steel provided that the recrystallization necesary to produce Goss texture is effected at a temperature below the loop, i.e., below the temperature of transformation from alpha to alpha plus gamma iron.

in addition to all the foregoing, it has also been found that manganese sulfide is not very effective in promoting secondary recrystallization to Goss texture at these lower temperatures required by the presence of the gamma loop in the nickel-containing steels. However, it has been further found that Goss texture can be promoted by vanadium carbides, which can be eliminated from the final steel during the secondary recrystallization at relatively low temperature, thereby enabling optimum magnetic properties to be achieved. Thus, our invention comprises first, the replacement of manganese by vanadium and the replacement of sulfur by carbon, and, second, producing Goss texture at a temperature below the transformation point from alpha to alpha plus gamma iron on heating.

FIGURE 3 of the accompanying drawing shows graphically the way in which this transformation temperature varies with the nickel content in steels of different silicon contents. The urves D, E, and F relate to 4%, 5% and 6% silicon contents, respectively. The area on the lefthand side of each curve is that in which the steel is ferritic. Thus, to take an example, the 4% silicon and 4% nickel steel is ferritic up to the point X, i.e., up to about 930 C. Although curves for only three silicon contents are given, it will be understood that similar curves for steels of other silicon contents can be drawn from tests on specimens of such steels, as will be appreciated and understood by those skilled in the art.

The speed of secondary recrystallization increases with the temperature, and it is therefore desirable that the temperature of secondary recrystallization should not be too low. We rather surprisingly find, however, that in the steel containing 4% silicon and 4% nickel a temperature of about 850 C., i.e., in the range of 845 C. to 855 C., gives the best results. On the other hand, in a steel containing 6% silicon, in which the nickel content should be at least 9%, the temperature may not exceed 830 C. if the steel is to remain wholly ferritic, and we prefer not to work below 800 C. In any event, the temperature should be at least 850 C. and advantageously does not exceed about 950 C., e.g., not in excess of about 930 C.

Considering the composition of the steel further, manganese is present, if at all, only .as an impurity, say not exceeding 0.02%. Sullfiur is now no longer required and, as an impurity, is kept as low as possible, say, below 0.003%. Broadly, the purer the final steel, except in respect of its essential elements, the better the final magnetic properties will be.

To be effective, the vandaium must be present at precipitated carbide during the secondary recrystallization. It may be precipitated in the course of the hot rolling and annealing steps or the steel may be heated for this purpose only immediately before the secondary recrystallization. The vanadium carbide appears from X-ray examination to be mainly V C It is a particular advantage of the use of vanadium that its forms carbide readily with the carbon in the steel and that this carbide is readily decomposed at the relatively low temperature of secondary recrystallization. Other carbides which might be used as inhibitors of primary recrystallization, such as titanium carbide and niobium carbide, are stable at the temperatures in question.

The carbon content is between 0.02% and 0.06%, and preferably is 0.03%, in the initial steel. Preferably, there is enough vanadium to combine with all the carbon but there is no advantage to be gained in including more vanadium than this. Having regard to these considerations, the vanadium content may be from 0.05% to 0.3% and is preferably from 0.08% to 0.15% in steels containing from 0.02% to 0.03% carbon.

Thus, in summary, the initial steel contains from 3.5% to 6.5% silicon, from 2% to 12% nickel, the nickel content being high enough to render the steel rollable at C., from 0.02% to 0.06% carbon and from 0.05% to 0.3% vanadium, the balance being iron except for impurities. The impurities such as sulfur should be kept as low as possible.

Two examples will now be given.

EXAMPLE I Steel containing 4% nickel, 4% silicon, 0.015% vanadium and 0.03% carbon, the balance being iron except for impurities, is hot rolletd at 1,000- C. into sheet 0.080 inch thick and annealed for 10 minutes at 900 C. The sheet is then cleaned by shot-blasting. The sheet is next reduced in thickness by rolling at 100 C., the metal being immersed in boiling water before each pass, with intermediate annealing steps, each comprising heating at 850 C. for 10 minutes in hydrogen, when the steel is 0.040 inch and 0.025 inch thick. In this way, the sheet is brought down to about 0.014 inch thick. The steel is next heated in an inert atmosphere for 15 minutes at 700 C. to precipitate the carbides. The steel is then heated in dry hydrogen for 24 hours at 850 C. and in the course of this heating Goss texture is produced and the vanadium carbide is decomposed, the vanadium going into solution in the steel and some carbon being removed by the hydrogen. The sheet can be further decarburized by annealing in wet hydrogen for 30 minutes at 750 C.

EXAMPLE II In this example the treatment is simplified; the anneal after the hot rolling is eliminated and no specific annealing treatment is given to precipitate the vanadium carbide. Further, by the use of wet hydrogen during the secondary recrystallization treatment, the need for a final decarburizing treatment to remove the carbides is eliminated.

Steel containing 4% nickel, 4% silicon, 0.10% vanadium :and 0.03% carbon, the balance being iron except for impurities, is formed by hot rolling at 1,000 C. into sheet 0.080 inch thick. The sheet is then cleaned by shot-blasting. The sheet is next reduced in thickness by rolling at 100 0., being immersed in boiling Water before each pass, with intermediate annealing steps, each comprising heating at 850 C. for minutes in hydrogen or cracked ammonia, when the steel is 0.040 to 0.025 inch thick. Secondary recrystallization to Goss texture, and at the same time carbon removal, is effected by annealing in wet hydrogen (dew point C. to minus 10 C.) for 24 hours at 850 C.

Grain-oriented steel containing 4% silicon and 4% nickel produced according to the invention has high saturation induction of about 19,400 gauss and has a watts loss per pound at 50 cycles per second less by about 10% than that of nickel-free 3.2% silicon grainoriented steel. At high frequencies, the advantage as shown by watts loss is more pronounced.

The present invention is of particular benefit in the production of grain-oriented silicon steels, the magnetic characteristics of which render them most useful in power applications, including transformers, motors, generators, etc. A common commercial application includes transformer cores formed of laminations of the grainoriented steels contemplated herein.

Although the present invention has been described in conjunction with preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. In the production of gnain-oriented silicon steels in which secondary recrystallization is effected to achieve Goss texture, the method of producing Goss texture in a steel sheet containing from 3.5% to 6.5% silicon, from 2% to 12% nickel, the nickel being present in an amount in which the minimum nickel content is correlated with the silicon content in accordance with curve A of FIG. 1 and suflicient to render the steel rollable at 100 C. from 0.02% to 0.06% carbon, and from 0.05% to 0.3% vanadium, the balance being iron except for impurities, which comprises effecting secondary recrystallization and at least substantial decomposition of the precipitated vanadium carbides in the steel by heating it at a temperature below that of the transformation point of alpha iron to alpha plus gamma iron.

2. A process as set forth in claim 1 in which the steel contains 0.02% to 0.03% carbon and from 0.08% to 0 .15% vanadium.

3. A process as set forth in claim 2 in which the secondary recrystallization is effected in an atmosphere of wet hydrogen.

4. In the production of grain-oriented silicon steels in which secondary recrystallization is effected to achieve Goss texture, the method of providing Goss texture in a steel sheet containing from about 3.7% to about 4.3% silicon, about 3.7% to about 4.3% nickel, about 0.02% to about 0.06% carbon, about 0.05% to about 0.3% vanadium, with the balance being iron except for impurities, which comprises effecting secondary recrystallization and at least substantial decomposition of the precipitated vanadium carbides in the steel by heating it at a temperature of at least about 750 C. but below the temperature at which transformation of alpha iron to alpha plus gamma iron occurs.

5. The method set forth in claim 4 in which the steel contains about 0.02% to about 0.03% carbon.

6. The 'method set forth in claim- 4- in which the temperature of secondary recrystallization is from about 845 C. to about 855 C.

References Cited UNITED STATES PATENTS 2,209,684 7/1940 Crafts.

3,096,222 7/1963 Fiedler 148-111 XR 3,147,158 9/1964 Fiedler.

3,184,346 5/1965 Fiedler.

3,214,303 10/1965 Fiedler 148l11 3,238,073 3/1966 Clark 148-3155 XR 3,239,332 3/1966 Goss 1481 11 XR 3,278,348 10/19'66 Foster l48110 233 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,446,680 Dated September 18, L969 Inventor(s) CHARLES ALFRED CLARK, RONALD JOHN BUTT 8: JOHN JEFFERSON MA;

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

w Column 3, line 70, for "urves" read --curves--. Column A, line 16, for "850C." read --750C.--. Column 4, line 58, for "0.015%" read "0.15%".

SIGNED AN'U SEALED M Meet:

Fletcher wnmmr E SCif-IUYLER JR 0mm 00111111551101 1151 of Patent!

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