US2358788A - Production of silicon steel of uniformly low core loss - Google Patents

Description

Patented Sept. 26, 1944 PBODUGIIO'N OF SILICON STEEL OF UNIFORMLY LOW, CORE LOSS Franklin, and John M.

Victor W. Carpenter,

Middletown,

assignors to .The American Jackson, Butler, Pa.,

Mill Company, Middletown,

tion of Ohio Ohio, and Jack E. Lucas.

Rolling Ohio, :1. corpora- No Drawing. Application August 3, 1940,

Serial No. 351,126

9 Claims.

Our invention relates to the'production of silicon steel for electric uses and has for its principal object the attainment of uniformly low core loss in low carbon silicon steels without si nificant increase in cost.

In the copending application of Victor W. Carpenter entitled Method of producing silicon steel sheet or strip, 1936, now Patent No. 2,236,519, issued April 1, 1941, there are set forth processes for the production of silicon steel characterized broadly by a decarburizing treatment between the hot rolling and cold rolling stages, which treatment is a box annealing of the material in the presence of the hot mill scale. This treatment is inexpensive and is effective in reducing the carbon content to .015% or lower irrespective of the starting carbon in commercial silicon steels. In material of such low carbon content a final box annealing can be dispensed with, since the grains grow so rapidly that the development of the desired magnetic properties can be achieved in a commercial continuous anneal of the single strand type. The treatment has come into use Ifoor many grades of silicon steel manufactured Most cold rolled silicon steel is sold on a core loss guarantee. Each commercial grade is made from a selected silicon grade of bar stock, by a series of operations appropriate to the particular result desired. In spite of the close adherence to standardized practices it has been found that variables in the processing or analysis which are not easily controlled may influence the final product to such an extent that certain batches of material may not meet the core loss guarantee.

' After examination of such batches of material over a long period of time, we

other factors have been calculated to a comhave found, when parable basis, that the difiiculty accompanies too small a grain size in the finished product. We therefore came to the conclusion that greater uniformity in achieving the lower core losses which we obtain, implied 'a variation in pro-. cedures to obtain a larger grain size on the average;--or, putting it another 'way, greater consistency in coreloss results would follow greater consistency in the production of a large I the finished product.

grain size in 7 known that very large grains may 'It has been be obtained in cold rolled silicon steel by strain rolling." This is essentially a finishing operation involving annealing strip when it is within a few thousandths of an inch of final gauge, cold rolling it to gauge, and

Ser. No. 60,347, filed January-22,

' size, the largerwill be .a working of the piece.

was possible without strain rolling? but which would be available for substantially all silicon steel grades because of relatively slight cost. The known operation of strain rolling was therefore not seen as available for our purpose. We have determined, however, that the grain size of the hot-rolled material (or hot-rolled material which has been annealed) will determine the ultimate grain size in'the cold rolled and annealed product. The larger theoriginalgraln the final grain size within certain limits. Thus, if the original hot-rolled thin bar could be made structure, uniformly, we believed we would secure the desired improvement and uniformity in the final product. Experience the case. 1 There are several methods by which a larger grain structure may be secured in the thin bar.

-A higher annealing temperature following the hotrolling will give larger grains. Also, if the hot rolling is finished at such a temperature as to leave considerable strain in the thin bar, grain growth will 'occur during the subsequent box anneal. Conversely, it is possible to finish the hot be discussed. We find .that good results can be.

more certainly attained and controlled by introducing strain into the hot-rolled bar afterthe hot rolling so that large grains will grow during the box anneal. There are various methods of accomplishing this purpose; but they all involve The one or more passes through a scale breaker, and this will be found to introduce sufiicient strain to produce useful grain growth or the piece may be strained after the hot rolling by passing it through an ordinary cold mill. Both of these procedures can be carried on so as not to remove the cold rolled sheet or I the hot mill scale from the hot-rolled part whereby the eflicacy of the following box anneal as a decarburizing treatment is not impaired. The

it. Strain rolling is exto have a larger grain has shown that this is piece may be given I ing on the first pass, would be strained on the.

second pass. The scale breaker treatment is somewhat less expensive than the cold rolling;

. but the use, of the mill for straining results in more uniform grain size.

We have. examined microscopically materials which have been strained by being passed through a scale breaker after the box anneal whichfollows. These materials were found to have large grains on each surface, which extended toward the midthickness of the bar about one-fourth of the way through the bar. Thus the box annealed bars made in this way contained large grains which comprised approximately one-half of the total thickness of the bar. The grains at the midthickness were of the size which would be obtained in. the box annealed bars without the straining. Nevertheless, the amount of material characterized .by large grains in the box annealed product was found sumcient to produce the desired large grains after the complete process of making the silicon steel sheets had been gone through. When the pieces strained by cold rolling are examined, the grain growth due to straining is found to be much more general and uniform throughout the thickness of the piece. a

As an outline of a general process which we heretofore carried'out, -we hot-roll silicon steel to a gauge say from,.06 to .10 inch. We then box anneal the strip in the presence of the mill scale whereby to decarburize it. Thereafter we pickle the strip, cold roll it to the final gauge, and open anneal it. This process as modified in accordance with the present invention may be summarized as follows: Hot roll to from .06 to .10 inch, introduce a critical amount of strain into the bar, box anneal, pickle, cold roll to final gauge, and open anneal. It will be noted that we have not added significantly to the cost of the process; but the uniformity of results and general improvement in results which we secure is marked and valuable.

As to the various factorsaffecting grain size of the hot-rolled material, all of these have a certain importance .and combinations of them are contemplated by our invention. Even where clnef reliance is placed upon a definite'working of the metal after it has cooled from the hot rolling temperature, the other factors of conv proper amount of working and a proper annealing temperature in view of the other factors for the desired grain growth. For example, using a medium silicon of around 2% we have found that around 7 to 8% reduction will result in a rolled gauges of silicon steel that are now being used. The essential limitation upon the hotrolled gauge is that of the power and capacity of the hot mills. Moreover, our invention applies to hot-rolled thin bars or the like of any silicon content, which are usable to produce cold rolled strip. The silicon content of the material also is a factor which has some bearing upon the amount of strain necessary to produce large grains.

The application ofour new process results in an enlargement in grain size over the same process with the new step omitted. It is not to be understood that it will produce grains quite as large as those resulting from the use of the more expensive strain rolling" process which has been described. The improvement resulting from the use of our process, however, is of such an order as to significantly enhance the magnetic properties of the material.

- 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 of producing silicon steel sheet or strip characterized by enlarged grain size, which comprises hot-rolling silicon steel to an intermediate gauge, introducing a critical strain by cold work into the hot-rolled material, box

' annealing it, thereafter cold rolling it to final gauge with a large percentage reduction substantially in excess of percentage reductions for critically straining it, and finally heat treating the silicon steel, to develop its magnetic properties and a large grain size.

2. A process as claimed in claim 1 wherein the introduction of I the strain is accomplished by passing the hot-rolled material througha scale breaken,

3.-The process as claimed in claim 1. wherein the strain is introduced into the hot-rolled material .by giving it a pass through a cold mill prior to the initial annealing.

'4. The process as-claimed in claim 1 wherein,

the strain is introduced into the hot-rolled material by giving ita pass through a cold mill prior to the initial annealing, the hot-rolled material as passed through said cold mill bearing the hot mill scale on its surfaces.

5. A process of producing low carbon silicon steel or strip which comprises hot-rolling silicon steel to an intermediate gauge, cooling the silicon steel, introducing a critical strain by cold work into the silicon steel, and thereafter boxannealing the silicon steel without removal of the hot mill scale, thereafter cold rolling the silicon steel to final sheet gauge by producing therein a cold rolling reduction substantially in excess of percentage reductions for critically straining it, and heat treating it to develop its magnetic properties and a large grain size.

6. A process of producing low carbon silicon steel sheet or strip which comprises producing hot-rolled silicon steel of an intermediate gauge characterized by critical strain productive of a large grain size upon annealing, annealing said silicon steel in the presence of the scale resulting from hot-rolling, whereby to decarburize it, pickling the'silicon steel, cold rolling it to final sheet gauge by producing therein a cold rolling reduction substantially'in excess of percentage reductions for critically straining it, and finally an-.-

nealing the silicon steel to develop its magnetic properties and a large grain size.

7. A process of producing silicon steel sheet or strip which comprises hot-rolling silicon steel to a gauge of substantially between .06 and .10 inches, introducing critical strain into the hot-rolled silicon steel, box annealing it, pickling it, cold rolling it to sheet gauge with percentages of cold rolling reduction substantially in excess of percentages for critically straining it, and thereafter open annealing it to develop its magnetic properties and a large grain size.

8. The process as claimed in claim 7 in which the box anneal is carried on in the presence of the hot mill scale so as to decarburize the silicon steel as well as cause grain growth therein.

9. A process of producing low carbon silicon steel sheet or strip of enlarged grain size, which comprises producing hot rolled silicon steel 0!.

an intermediate gauge of approximately from .06

to .10 inch, cooling the hot rolled silicon'steel to cold rolling temperatures, cold rolling it without removing the hot mill scale from its surfaces, and with only sufllcient elongation to produce a critical strain in the silicon steel, thereafter box annealing the silicon steel still in the presence of the hot mi'llscale, whereby to decarburize it by.

reducing the carbon therein froma higher value to approximately a maximum of .015%, pickling 'the silicon steel, cold rolling it to final sheet ;vrc'roa' w. CARPENTER.

JOHN M. JACKSON. .mcx E. LUCAS.