US2412041A - Process for flattening silicon steel sheets - Google Patents

Description

Patented Dec. 3, 1946 PROCESS FOR FLATTENING SILICON STEEL SHEETS Carl E. Giflzord, Zanesville, Victor W. Carpenter, Franklin, and Lowell L. Cook, Zanesville, Ohio, assignors to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application March 28, 1941, Serial No. 385,756

Claims. 1

There has been recently a considerable tightening in the requirements for flatness in sheets of electrical steel. It will be understood that the flatness characteristics of magnetic sheet metals vary considerably in accordance with the mode of manufacture of the sheets, so that diiferent standards of flatness have been thought to apply to differently processed materials. While cold reduced strip has been developed to the point where it is commonly quite flat, light gage, hot rolled sheets are at best somewhat wavy and consequently are at a commercial disadvantage for some applications.

It is an object of our invention, therefore, to provide a means whereby the flatness characteristics of hot rolled sheets may be usefuly improved. While the degree of improvement will vary somewhat with the flatness of materials before they have been given our treatment, it is nevertheless, following the general object set forth above, an object of this invention to provide a treatment which will impart flatness characteristics substantially equal or better than the best flatness hitherto produced, to this material hitherto characterized by notable lack of flatness, i. e. materials hot rolled to gauge.

It is an object of our invention to provide a flatness treatment which in itself is not only relatively inexpensive, but which may be practiced upon magnetic materials without impairing the magnetic characteristics previously developed therein by other treatments. Further it is an object of our invention to provide a treatment which is capable, in some modes of manufacture, of being substituted for a final heat treatment for the production of magnetic characteristics. In this way further economies may be effected, as will be set forth hereinafter.

Our invention has been found applicable to all commercial grades of hot reduced electrical steel sheets irrespective of the quantitative silicon content, or the content of other alloying ingredients, and the particular routing of the steel does not form a limitation upon the invention herein described.

The objects set forth above, and others which will be pointed out hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish bythat treatment and have not come into wide commercial usage due to their resulting in a material of extremely poor flatness. It was hitherto supposed that the only way of increasing the flatness characteristics of materials poor in these respects was by giving them a carefully controlled box anneal. Even here, however, the flatness characteristics were not fully satisfactory, and furthermore it is not always desirable, either from the standpoint of resultant magnetic and physical characteristics or from the standpoint of economy, to conclude certain manufacturing processes with a box anneal.

We have discovered that under certain circumstances an open annealing treatment may be caused to effect a marked improvement in the flatness characteristics without involving undue expense; that our treatment, where desired, may be practiced upon otherwise finished materials without impairing their previously developed magnetic characteristics; and that where an open annealing treatment is desired as part of the processing treatment of the steel for the sake of other characteristics (such as ductility, magnetic characteristics and the like), the open annealing for the production of these desired results can be so carried on as to respond to the invention herein set forth without diminishing its efiectiveness for the said results.

We have found that if a heat treatment in an open annealing furnace is carried on, on the materials while they are subject to a controlled tension, greatly enhanced flatness will be the result without detriment to the materials in any way. While it is possible, it would be very difficult and costly to tension single hot rolled sheets at an elevated temperature. If the sheets are carefully welded together, preferably end to end to form a long band, the means of applying tensiOn during a continuous annealing becomes relatively simple.

In forming a continuous supply from individual sheets or strips we prefer accurately to butt weld the sheets or strips together by the procedures and using the apparatus of the, following patents: U. S. Letters Patents Nos. 2,172,080, 2,172,081, 2,219,493, 2,175,615, 2,175,616, 2,196,941, 2,282,611 and 2,254,314.

By these procedures, we secure very long lengths of silicon steel characterized by welds at intervals, the welds however being substantially no thicker than the welded sheets, and being so perfectly formed that the steel in the endless strip is useful for'all of the purposes to which the sheet material itself can be put, quite irre- 3 spective of the position of the welds. Welded strip formed in this way, however, has not hitherto been characterized by great flatness.

Moreover, it would normally be supposed that the strains introduced into the strip by .thewelding would be productive of a marked decrease in flatness if the welded strip were subjected to an open annealing, We have found however, that by subjecting the welded strip to tension during the heat treatment we can improve the flatness to the point where it exceeds that produced by a good box anneal.

The tension required is not great. Indeed with certain kinds of materials, where a fairly long furnace is used, the desired degree of flatness may be produced by the weight of the strip only, the strip being supported both as it enters and as it leaves the furnace. We prefer however, so to operate as to apply a positive tension to the stripes it passes through the furnace. This is readily accomplished by providing feeding devices for the strip both at the entrance and at the exit ends of the furnace and driving the feeding devices in a synchronized way so that the exit feeding device operates very slightly faster than the entrance feeding device, Pinch rolls may be used; and they may be driven at the required related speeds by a connecting chain or shaft drive. In a type of open annealing furnace currently being used by us, continuous supports have been eliminated, and the welded strip is supported upon driven rollers spaced about two feet apart. By means of synchronized pinch rolls, we apply to the strip sufficient tension to keep it from sagging'to any great extent between the interspaced supports, and such tension has been found" suincient to produce excellent flatness.

Such tension is obtained by operating our exit rolls at a peripheral speed, usually 0.1 to 0.5% faster than the-entrance rolls.

As indicated, the amount of tension required is not great. The strength of the material at the elevated temperature is low; for example, 4.5% silicon steel has a tensile strength of about 15,000 pounds per square inch at 1200 F., 3,000 pounds at 1500 F. and 1,500 pounds at 1800 F. While more tension is required than merelythat sufficient to move the strip when supported substantially throughout its length on low friction supporting means, yet for some requirements the weight of the material throughout a, fairly long span is often sufiicient. One reason we believe that the amount of tension required is relatively low, is that this type of material at elevated temperatures has no definite yield point but begins to creep at relatively low stresses, Consequently the tensile strengths cited are only approximate since the material permanently elongates somewhat even with very low tension if enough time i given.

Thus the amount of tension required will vary with the composition and gage of the material, temperature to which the strip is heated, and the length of time at which it is held at temperature. t will also vary somewhat with the degree of waviness of the material -to be flattened. For best results we have discovered that under whatever conditions the process is used, the tension must be sufiiciently great to permanently elongate the material slightly. For example, we commonly obtain a permanent elongation of 0.15 to 0.3% on one grade of material when given our treat ment.

While not wishing to be limited by a'ny-theory, we believe that the zlack o-f flatness lof light :ga'ge,

hot rolled sheets is due to certain areas or surfaces being shorter than they should be, and that the flattening action is due to the effect of the tension in elongating the short areas until they are all the length required in a .flat sheet and the stresses set up by the lack of uniformity are relieved.

An exact rule for the determination of the correct amount of tension to be employed to flatten all wavy materials using all types of annealing apparatus cannot be arrived at on account of the interplay of the many variables, but with any given material, at any given temperature, it will be within the skill of the worker in the art to select and apply an average tension which is sufficient to give the flatness desired without being great enough to produce longitudinal wrinkles or corrugations in the strip, or pulling it in two,

The temperature of the heat treatment to produce flatness is not at all critical; for example we have used temperatures as low as l200 In most cases the-temperature will be governed by a consideration of the other results expected to be produced, or desired, from the open annealing. Where the primary purpose of the treatment is to produce flatness, it is well to choose a temperature which is below the graphite solubility temperature, so that the treatment ,does'not have the result of bringing the graphitic carbon into solution. Under these circumstances, the magnetic properties previously developed in the material will not be impaired, and since there has not been a resolution of the graphitic carbon, the treatment will not be productive of magnetic aging.

For example, when operating upon a silicon steel containing say from 4.5 to 5.0% silicon, which steel has already been given a high temperature box anneal for developing the required core loss and permeability, We carefully weld individual sheets together and pass them under tension through an open annealing furnace kept at 1490 to 1500 F. or thereabouts, holding the steel in the hot zone around one and one-half minutes. Such a treatment greatly improves flatness while not harming the other properties of the material and in some cases improving them.

In those cases where the carbon content is quite low, such as when no carbide or graphite particles are visible on microscopic examination, much higher temperatures can be satisfactorily used. 'Or if flatness is the only consideration then the maximum permissible temperature is pretty muchdetermined by mechanical factors.

On the other hand, in processes for producing magnetic steels, where an open anneal is desirable for other reasons as one of the steps or as the final step of a particular routing, temperatures appropriate to the said step may be used and our flatness attained by the use of proper amounts of tension. For example, when operating with a silicon steel of a silicon content below say 4.0%, which silicon steel has been decarburized by box annealing it at an intermediate gauge in the presence of the hot mill scale (in accordance With the teachings of Patent 2,242,334 to Carpenter) and has been reduced to gauge, wecarefully weld thesheets into a strip and givethe strip a final open annealing .under tension at around 1500 to '2100 F. This treatment not only produces the desired flatness but is an integral step in the production-of the desired magnetic properties.

The tension bears a certain relation to gauge and temperature; ?but with material of any given gauge, having selected a temperature for treatment as taught above, the skilled Worker can then select a tension to give him the desired result. As to the duration of the heat treatment, our investigations have shown that no great holding time is necessary if the material is brought to a softening temperature during the application of the tension. The length of time the material is in the furnace and subjected to the temperature thereof will vary of cours rimarily with the length of the furnace and the speed of travel of the strip. The length of time it takes to bring material up to temperature is of course affected by the gauge. With silicon steel of ordinary transformer gauges the material will usually reach temperature within one-half to one minute. We have found it preferable not to cool the strip too abruptly as by bringing it immediately out into the air from the hot zone.-- We prefer to attach to our furnaces a cooling hood whereby the temperature drop may be made somewhat more gradually. The atmosphere within the furnace does notaffect the flatness, hence forms no limitation on our process. The atmosphere may be chosen in accordance with its effect upon other qualities such as brightness, core loss, ductility and the like. We prefer to use a neutral or reducing atmosphere to substantially prevent any scaling of the strip.

It will be seen that our process is much more effective than those processes employing, for example, an open anneal and a light cold rollin prior to a box anneal, because the products of those processes have hitherto been characterized by lack of flatness. Thus where the flatness of the product resulting from a box anneal to develop magnetic properties is not satisfactory for a given use, the sheets may be weldedtogether and passed under tension through an open annealing furnace merely to increase their flatness. Again where an open anneal after a box anneal is desired for any other purpose, the open annealing may be carried on at the appropriate temperatures for these results, but under tension in accordance with the present teachings. Yet again, as where the material has, for example, been decarburized at an intermediate gauge and then hot rolled to final gauge so that a final pen anneal is all that is required to develop its magnetic properties, the said final open anneal may be made productive of flatness in accordance with the present teachings.

The examples which we have given are not limiting since, as we have explained, our treatment for the production of flatness can either be an added treatment for the specific purpose, or can take the place of an annealing already a part of the desired process for the production of the steel. Our treatment will thus be most generally applied to material which has been reduced to final gauge.

Modifications may be made in our invention without departing from the spirit of it.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. A process for increasing the flatness of finished silicon steel in strip form, made by joining together pieces of lesser length with transverse butt welds, and which is characterized by lack of flatness, which comprises bringing the sheet material to a temperature at which it becomes soft enough for the relief of strains but below the graphite solution temperature, and while at said temperature subjecting the material to tension sufilcient to flatten it, and produce a slight overall elongation in the material, without producing in the'material any detectable change in the crystal orientation.

2. A process for increasing the flatness offinished silicon steel in strip form, made by joining together pieces of lesser length with transverse butt welds, and which, in finished form, is characterized by lack of flatness, which comprises bringing the material to a temperature at which it is soft enough for the relief of strains but below the graphite solution temperature, and subjecting the material to tension sufficient to flatten it and produce therein a slight over-all elongation while at said temperature and while cooling said material to normal temperatures at a rate slower than open cooling in air at room temperature, without producing in the material any detectable change in the crystal orientation.

3. The step of increasing the flatness of finished, hot rolled silicon steel sheets which comprises joining the sheets end to end to form a band, and subjecting the band to a softening temperature of the order of 1200 F., and higher, but insufficient to cause graphite solution, and concurrently subjecting the band to tension sufficient to increase flatness and produce a slight over-all elongation in the band, without producing in the material any detectable change in the crystal orientation.

4. The step of increasing the flatness of wavy, hot rolled silicon sheet steel which comprises joining sheets end to end to form a band and subjecting the steel to a softening temperature while tensioning the band sufiiciently to increase flatness by passing the band through a continuous open annealing furnace including a cooling hood and positively feeding the hand both into and out of the furnace at speeds interrelated to produce tension in the strip and a slight over-all elongation therein, the value of the tension employed being not greater than that required to eliminate substantial sag in the material in a span substantially equivalent to the furnace length.

5. The step of increasing the flatness of wa y silicon sheet steel which comprises joining sheets end to end by butt welding to form a band, and passing the band so formed through an open annealing furnace while controlling the entrance speed of the band into the furnace and its exit speed therefrom to produce a tension and elongation in the band, while subjecting the band to a softening temperature, the value of the tension employed being not greater than that required to eliminate substantial sag in the material in a span substantially equivalent to the furnace length,

6. A process as set forth in claim 5 in which the temperature to which the silicon steel is heated is above 1200 F. but less than the graphite solubility temperature of the silicon steel.

7. A process as set forth in claim 5 in which the silicon steel prior to the treatment for flatness is box annealed, and in which the temperature to which the silicon steel is heated during the treatment for flatness is above 1200 F. but somewhat less than the graphite solubility temperature of the silicon steel.

8. A process for flattening silicon steel which comprises forming silicon steel sheets characterized by transverse waviness into a strip like supply of indefinite length and sheet gauge by butt welding, and passing the said supply through an open annealing furnace at a temperature between 1200 and 1500 ,F., providing interspaced :feeding devices acting on the supply as it passes through the furnace, and operating said feeding devices at difierential speeds so related as to prevent cumulative sagging of the supply if it were unsupportedin said furnace, whereby to produce a tension substantially limited to the tension which would be exerted on the material by its .own weight if unsupported in said furnace.

9. A process for flattening silicon steel which 10 comprises forming silicon steel sheets characterized by transverse waviness into a strip like supply .of indefinite length and sheet gauge by butt welding, and passing the said supply through an open annealing furnace at a temperature of 15 around 1200 to 1500 F. while subjecting the supply to tension limited to an amountsubstantially suilicient to compensate for sagging of thematerial under its own weight at the said temperatures if unsupported in a span of furnace length, the v20 10. In a process of producing silicon steel inpcludingthe step of simultaneously producing the supply of indefinite length, passing said supply through an open annealing furnace and subjecting it therein to a temperature of between sub-- stantially 1500 and substantially 2100 F. while .tensioning the supply to produce flatness therein by positively controlling its entrance'and exit speeds to substantially those differential speeds which would be required to prevent cumulative sagging of the said supply under its own weight at the said temperatures if unsupported in the said furnace.

CARL E. GIFFORD. VICTOR W. CARPENTER. LOWELL-L. COOK.