US3130091A

US3130091A - Non-oriented silicon-iron sheet stock and process of making it

Info

Publication number: US3130091A
Application number: US731116A
Authority: US
Inventors: Victor W Carpenter; John M Jackson
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1958-04-28
Filing date: 1958-04-28
Publication date: 1964-04-21
Anticipated expiration: 1981-04-21

Description

United States Patent On ice 3,130,091 Patented Apr. 21, 1964 3,130,091 NON-ORIENTED SILICON JRQN SFEET STGCK AND PROCESS 9F MAKING IT Victor W. Carpenter, Franklin, and John M. Jackson,

Middletown, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Filed Apr. 28, 1958, Ser. No. 731,116 12 Claims. (Cl. 148-111) The invention relates to the production of non-oriented silicon-iron, by which is meant a magnetic sheet stock not characterized by such a degree of preferred orientation as would make for substantial differences in permeability in the straight-grain as compared with the crossgrain directions. Non-oriented silicon-iron sheet stock is especially useful in rotating electrical machinery, although it has other uses as well.

Hitherto, there have been two generally recognized methods for making such stock. The silicon-iron can be hot rolled to gauge and given a heat treatment. Or it can be hot rolled to an intermediate gauge and then cold rolled to final gauge and heat treated. Each method has its advantages and disadvantages. The material which is hot rolled to final gauge can be made only in sheet form. However, particularly when subjected to a high temperature anneal, it will be characterized by a relatively large grain size, which makes for a lowered core loss. The material which is cold rolled to final gauge can be made in long lengths in coil form; but tends to be characterized by a relatively small grain size unless subjected to temper rolling and a subsequent grain growth anneal, which are matters of additional expense.

Non-oriented silicon-iron of either form will normally require decarburization at some stage to minimize hysteresis losses; but decarburization treatments as ordinarily practiced do not make for large grain size.

A silicon-iron which is finished and has its magnetic properties fully developed in the plant of the steel producer is known as fully processed steel. The purchaser will stamp laminations from this material; but the stamping operation usually sets up strains in the material which are detrimental to the best magnetic properties, so that the purchaser will have to subject the laminations to a stress relieving anneal before he can use them. It has been realized that a saving could be made if the purchasers anneal could accomplish a part of the processing; and there has been a growing tendency in this field toward the production of semi-processed stock which can be sold as such.

A semi-processed stock is one in which the magnetic properties are not completely developed during the processing by the producer, and by definition should be, as produced, at least one magnetic grade lower than the fully processed stock. The relatively greater core loss of semi-processed stock may be the result of inadequate grain growth or incomplete decarburization or both; but a proper semi-processed stock must be of such character that its magnetic quality will be raised to that of the fully processed stock when punchings are subjected to an anneal within the capabilities of the average purchaser. The purchaser will anneal his punchings in a muffle or like apparatus in which the atmosphere can be controlled if need be; but the purchaser is normally equipped to anneal only at a relatively low temperature, say, 1400 to 1600 F.

The problem is further complicated by the fact that the material, in whatever condition it is sold, must give a satisfactory die life, since the first operation performed upon it by the purchaser will be the stamping of laminations.

It is an object of the invention to provide a process for the manufacture of cold rolled non-oriented siliconiron in :which a large grain size can be dependably attained without the expense of a temper rolling to promote grain growth in the subsequent anneal.

It is an object of the invention to provide a non-oriented silicon-iron sheet stock which both in the fully processed and semi-processed conditions will have long and satisfactory die life characteristics.

It is an object of the invention to provide a semiprocessed, non-oriented silicon steel which either initially has or can readily be converted into a product having large grain size by a purchasers anneal at or around 1400 to 1600 'F.

It is an object of the invention to provide a process for the manufacture of non-oriented silicon-iron sheet stock of desired magnetic properties which is simpler and cheaper than those hitherto known.

These and other objects of the invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished in that procedure and product of which certain exemplary embodiments will now be described.

By silicon-iron is meant a material containing substantially 0.5% to about 3.8% silicon. Aluminum may be present in quantities up to about 0.5%. The carbon content, from the practical standpoint, may vary from about 0.02% to about 0.08%. In addition to silicon, aluminum and carbon, the alloy may contain such amounts of other elements, e.g., manganese, phosphorus, sulfur and the like as are usual in silicon-irons in view of their method of manufacture. The balance of the \alloy will be substantially all iron. The analysis given above relates to the material after processing has started, i.e., it is not a ladle analysis, but may be the analysis of the hot rolled material.

First, two exemplary routings will be given, after which the steps included therein will be discussed.

Example I .F ully Processed (1) Hot roll to an intermediate gauge.

(2) Pickle, or anneal and pickle.

(3) Cold roll to final gauge.

(4) Strand anneal at a temperature of substantially 1950 to 2200 F. in a decarburizing atmosphere, and pickle.

(5) Strand anneal in a decarburizing atmosphere at substantially 1500 to 1700 F.

Example II.Semi-Pr0cessed (1) Hot roll to an intermediate gauge.

(2) Pickle or anneal and pickle.

( 3) Cold roll to final gauge.

(4) Strand anneal at substantially 1950 to 2200 F. in

a decarburizing atmosphere, and pickle.

(5) Anneal at around 1400" to 1600 F. This anneal is performed by the purchaser, normally in a box or mufl'le and in a decarburizing atmosphere.

The hot rolling is done as is usual in the treatment of silicon-iron; and the intermediate gauge is chosen at such a value that the stock can thereafter be reduced to final gauge by a single part cold rolling, by which is meant such cold rolling as can conveniently be accomplished without any intermediate anneals, whether or not the material is passed once or a greater number of times through a single mill or a plurality of cold mills. Since silicon-iron sheet stock of different gauges is required for different purposes, the intermediate and final gauges do not constitute a limitation on the invention. By way of example merely, the material may be hot rolled to a gauge of about .07 inch, but this can be varied, particularly in view of the desired final gauge.

Good results may be obtained by preparing the hot rolled material for cold rolling by pickling. However, in general, improved magnetic qualities can be attained by annealing the hot rolled material. A preferable anneal is a strand anneal at a temperature above 1600 F. An atmosphere oxidizing to both iron and silicon is preferred in order to facilitate subsequent scale removal.

The cold rolling is also carried on in the usual fashion. There are no limits on the extent of the cold rolling other than the practical limitations inherent in reducing the material to gauge in one part as defined above. Exemplary, but not limiting, cold rolled gauges are .014 inch to .025 inch.

Steps (4) and (5) of the routings given above are of the greatest importance in this invention, and require to be discussed in detail. These steps form a combination in which, when they are carried on as hereinafter described, several actions occur:

(a) The carbon content is carried down to a desired low final value, i.e. a value not greater than about 0.01%,

(b) A material is produced which has a relatively large grain size,

A material is, produced which, either after step (4) in semi-processed stock, or after step in fully processed stock will give a good die life, and

(d) A material is produced as a result of step (4) which will be raised by at least one magnetic grade by the step (5) procedure, thus making possible the provision of semi-processed stock which may be sold after step (4).

These various factors are interrelated, as will be explained. First considering decarburization:

In United States Patent No. 2,287,467, in the names of the present inventors, there is taught a decarburization of silicon-iron in a strand anneal at temperatures ranging between about 1350" and about 1650 F. in a hydrogen bearing atmosphere containing a substantial amount of water vapor. This is an effective decarburization treatment widely used in making various grades of siliconiron; but it does not substantially increase grain size, and it yields a material which is difficult to pickle.

In the decarburization of silicon-iron, the rate of carbon reduction is rapid in the temperature range of 1350 to 1650? F. Above 1650 F.'the rate decreases until a temperature of about 1800 F. is reached; but above that temperature the rate increases again. Thus, it becomes possible to perform effective decarburization at the high temperatures of step (4), namely 1950 to 2200 F. The reasons why decarburization at high temperature is desired will be set forth later.

While decarburization should occur during step (4), it should not be carried to completion in that step. I There are several reasons for this. If the decarburization were carried out in an atmosphere reducing to iron and completed in step (4) of the above examples, the die life of the product would be prohibitively poorer by reason of the high temperature and the production of silica at the surfaces, which silica is diflicult to pickle away. By carrying on step (4) in an atmosphere which is oxidizing both to iron and to silicon, and by limiting the treatment so that only partial decarburization occurs, it is possible not only to produce a product which will pickle readily, but also to produce a product which will give good die life on punching, and can be improved by at least one magnetic grade in the step (5) heat treatment, making possible the production of good semi-processed stock. Moreover, where fully processed stock is being made, the additional decarburization occurring in step (5) will not impair the die life characteristics of the product.

In one of the mechanisms of grain growth as hereinafter explained, the ideal product after the completion of step (4) is one having low carbon surface layers, and a higher carbon interior.

The value to which the carbon content is reduced in step (4) may be'varied with the initial carbon content; but as a general rule the carbon content should not be lowered beyond about 0.015%

It may be noted that the pickled material which is the result of step (4) is what is sold to the purchaser as semi-processed stock in Example II. The pickled surface of the stock can readily be coated with insulative materials such as described in United States Patents 2,501,846 or 2,492,095 in the name of Gifford, or treated with any desired rust preventive formulation.

As to the matter of grain growth, it has been found that when the silicon-iron is treated in a strand anneal at the high temperatures of step (4), grain growth can either be effected or initiated through the occurrence of one or both of the phenomena next to be discussed.

First, if the carbon content of the silicon-iron is low enough, the grain size will increase during a strand anneal at temperatures of 1950 to 2200 F., just as grain growth will occur in any other low carbon iron at such temperatures. Since this usually requires an initial carbon content of about 0.02% or less, the condition is not readily attained unless the steel has been melted to this low carbon content or a previous decarburization has been practiced or unless the anneal is somewhat prolonged. However, it is possible to manufacture excellent material in this way.

Second, where the carbon content is higher, a different phenomenon occurs, which is explainable as follows: If a phase change can be brought about in the silicon-iron while it is held at a constant temperature, the formation of new grains will be initiated in the material. Such a phase change can be effected through the removal of some element during the heat treatment. silicon-iron is held at a temperature such that it is mostly in the gamma phase, and if a part of the iron is changed to the alpha phase during the maintenance of the same temperature, new grains will begin to grow. The change in the iron from the gamma phase to the alpha phase can be brought about by the removal of an alloying element, e.g., carbon.

Where carbon is the element removed to bring about the phase change, the new grains tend to grow inwardly from the surfaces of the sheet stock in a columnar fashion, because it is at and adjacent to these surfaces that the change in composition and hence the change in phase takes place.

It has been found that a large grain size in silicon-iron can be attained as a derivative of such columnar grain growth. In fact, by the methods herein outlined, a siliconiron sheet stock can be manufactured with larger grains than can be produced otherwise, excepting by the expe-, dient of critically straining the material and then annealing it.

The attainment of columnar grains is not alone due to the quantity of carbon in the material, but depends upon the elimination of sufiicient carbon to produce a phase change at a constant but high temperature. Thus, the conditions promoting columnar grain growth are the presence of carbon initially in a substantial amount, the use of a high annealing temperature, and the elimination of sufficient carbon at the high temperature by means of aldecarburizing atmosphere to produce the aforesaid phase c range.

The conditions are readily attained in silicon-irons of.

relatively high initial carbon content, say, about 0.05% or higher, throughout the entire range of silicon contents which has been given above. However, it is possible to obtain the same or a similar effect with lower carbon contents providing the silicon content is also low. Thus, columnar grains can be started in a silicon-iron containing 0.03% carbon or less if the silicon content is less than about 2%.

Whether the grain growth is produced as such in step (4) by subjecting very low carbon material to the high temperatures, or whether the formation of columnar grains is initiated therein in a relatively low carbon material having also a low silicon content, the subsequent decarburizing anneal, step (5), should be more carefully carried on in order to reduce the final carbon content to less than 0.01%. For this reason, stocks having initially the lower carbon contents are preferred to be treated in ac- Thus, if a sheet of cordance with Example I for the production of fully processed stock. Semi-processed stock is more readily and economically made from material having initially higher carbon contents, sa 0.05% or higher. However, semi-processed stock can be made by any of the procedures herein outlined.

An ideal product resulting from step (4) is one in which, as a result of the treatment carried on as described, the surfaces of the stock are characterized by columnar grains extending inwardly from the sheet surfaces, there being at the center of the stock a layer of higher carbon material not characterized by the columnar grains. The over-all carbon content of such a stock could range from about 0.015% to about 0.025%. With such a stock, completion of the decarburization at the lower temperature of step (5) will not only reduce the carbon to a value below 0.01% but also continue the grain growth as a derivative of the initial columnar grains.

In Example I the last noted step is a strand anneal at about 1500 to about 1700 F. in which the decarburization is completed. Thus in normal practice the carbon will be carried down to less than .0l%. The final step in Example H is an anneal at about 1450 F. practiced by the purchaser. It will normally be carried out in a decarburizing atmosphere.

The temperature differences noted in Examples I and II for step (5 are of significance only in that a purchasers anneal is likely to be carried on at a somewhat lower temperature than the corresponding anneal in the steel manufacturers plant. The essential results can be attained generally in the range of about 1400 to about 1700 F.

In the manufacture of fully processed material, as in Example I, the step (5) anneal should preferably be carried on in an atmosphere reducing to iron, because iron oxide on the surfaces of the stock might interfere with punching quality, although the relatively low temperatures of the heat treatment avoids the formation of large silica particles at the surface. On the other hand, in the treatment of semi-processed stock, the purchaser can employ, if desired, a decarburizing atmosphere which is oxidizing to iron, since the punching will already have been accomplished.

The skilled Worker in the art will recognize what is meant by atmospheres which are decarburizing, and reducing or oxidizing to iron and reducing or oxidizing to silicon; and reference may be made here to Patent No. 2,287,467 mentioned above. In general, hydrogen-bearing atmospheres, devoid of carburizing gases, can be controlled as to their oxidizing or reducing characters toward iron and silicon by controlling their moisture contents or dew points.

Modifications may be made in the invention Without departing from the spirit of it. The invention, having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

1. A process for the manufacture of non-oriented silicon-iron stock of sheet gauge, which comprises hot rolling to an intermediate gauge a silicon-iron containing not more than about 3.8% silicon and substantially 0.02% to 0.08% carbon, pickling the hot rolled material, cold rolling it to final gauge without intermediate annealing, and then subjecting the material to a strand annealing treatment at a temperature of substantially 1950 to 2200 F. in a decarburizing atmosphere whereby to remove part of the carbon therein while leaving a total carbon content in excess of 0.01%, pickling the stock, and thereafter subjecting it to an annealing treatment at a temperature of substantially 1400 to 1700 F. in a decarburizing atmosphere to reduce the total carbon content to a value less than about 0.01%.

2. The process of claim 1 in which grain growth is produced in the material in the first mentioned annealing by reason of a low carbon content of the stock during said annealing.

3. The process of claim 1 in which columnar grains are formed at the surfaces of the stock in the first mentioned annealing due to the removal of sufiicient carbon to produce a phase change at constant temperature.

4. A process for the manufacture of non-oriented silicon-iron stock of sheet gauge, which comprises hot rolling to an intermediate gauge a silicon-iron containing not more than about 3.8% silicon, and substantially 0.02% to 0.08% carbon, pickling the hot rolled material, cold rolling it to final gauge without intermediate annealing, and then subjecting the material to a strand annealing treatment at a temperature of substantially 1950" to 2200 F. in a decarburizing atmosphere whereby to remove a part of the carbon therein, while leaving an over-all carbon content in excess of substantially 0.01%, and to produce a material characterized by relatively large grains of low carbon content at the surface and relatively higher carbon content grains in the interior of the stock, and then pickling the said stock, the said stock having a good die life, the said stock being a semiprocessed stock capable of improvement in magnetic properties by at least one grade in a heat treatment at substantially 1400 to 1700" F. accompanied by decarburization to a total carbon value of less than about 0.01%.

5. The process claimed in claim 4 in which the hot rolled material is subjected to an anneal at a temperature above about 1600 F., and is pickled prior to cold rolling.

6. The process claimed in claim 1 in which the said annealing treatment in a decarburizing atmosphere reducing to iron, at a temperature of substantially 1400 to 1700 F. in which the stock is reduced to a carbon content less than about 0.01%, is performed as a strand anneal.

7. The process claimed in claim 1, including the steps of punching laminations from the said stock, and then performing the said anneal thereon at around 1400 F. to 1600" F.

8. A process for the manufacture of non-oriented silicon-iron stock of sheet gauge, which comprises hot rolling to an intermediate gauge a silicon-iron containing not more than about 3.8%% silicon, 0.02% to about 0.08% carbon, and not more than about 0.5% aluminum, the balance being all iron except for normal contents of manganese, sulfur, and phosphorus, pickling the hot rolled material, cold rolling it to final gauge without intermediate annealing, then strand annealing the cold rolled material at a temperature of substantially 1950 to 2200 F. in an atmosphere oxidizing to iron, whereby to remove a part of the carbon and produce a phase change from gamma to alpha iron at the surfaces of the material, the said phase change resulting in the production of new grains of columnar form at said surfaces, pickling the material, and finally subjecting it to a decarburizing anneal at a temperature between substantially 1400 and 1700 F. whereby to reduce the carbon content in said material to a value less than about 0.01% and to produce an enlarged grain size as a derivative of said columnar grains.

9. The process claimed in claim 7 in which the hot rolled material is subjected to an anneal at a temperature above about 1600 F. before pickling.

10. The process claimed in claim 1 wherein said hot rolled material is subjected to an anneal at a temperature above about 1600 F. and is pickled prior to cold rolling.

11. The process claimed in claim 4 including the steps of punching larninations from the stock and then performing an anneal thereon, the said anneal involving heating the stock to a temperature between substantially 1400 and 1700 F. in a decarburizing atmosphere.

12. The process claimed in claim 4, including the steps of punching laminations from the said stock, and then subjecting the said stock to an anneal in a decarburizing atmosphere at a temperature of substantially 1450" F., to the extent of reducing the carbon content of the steel to a value below substantially 0.01%.

References Cited in the file of this patent 8 Scharschu Jan. 4, 1938 Yensen et al Dec. 13, 1938 Williams Dec. 7, 1948 Jackson Dec. 26, 1950 Maxwell Oct. 2, 1956 FOREIGN PATENTS Great Britain Feb. 17, 1954

Claims

1. A PROCESS FOR THE MANUFACTURE OF NON-ORIENTED SILICON-IRON STOCK OF SHEET GAUGE, WHICH COMPRISES HOT ROLLING TO AN INTERMEDIATE GAUGE A SILICON-IRON CONTAINING NOT MORE THAN ABOUT 3.8% SILICON AND SUBSTANTIALLY 0.02% TO 0.08% CARBON, PICKLING THE HOT ROLLED MATERIAL, COLD ROLLING IT TO FINAL GAUGE WITHOUT INTERMEDIATE ANNEALING, AND THEN SUBJECTING THE MATERIAL TO A STRAND ANNEALING TREATMENT AT A TEMPERATURE OF SUBSTANTIALLY 1950* TO 2200*F. IN A DECARBURIZING ATMOSPHERE WHEREBY TO REMOVE PART OF THE CARBON THEREIN WHILE LEAVING A TOTAL CARBON CONTENT IN EXCESS OF 0.01%, PICKLING THE STOCK, AND THEREAFTER SUBJECTING IT TO AN ANNEALING TREATMENT AT A TEMPERATURE OF SUBSTANTIALLY 1400* TO 1700*F. IN A DECARBURIZING ATMOSPHERE TO REDUCE THE TOTAL CARBON CONTENT TO A VALUE LESS THAN ABOUT 0.01%.