US2083275A - Manufacture of silicon steel sheets - Google Patents

Description

' degree. Tr

aging, that is, they sho age, especially as Patented June 1937'.

s P TENT oFF -cs MANUFACTURE or srucon s'rnnr. snnn'rs Orlando E. Romig, Gary, Ind., assignmto Amer-i Plate Company, a corporacan Sheet and Tin tion of New Jersey No Drawing. Application August 29, 1930, Serial 1 Claim. (Cl. 148-12) 7 This invention relates to the manufacture of silicon steel products and although not limited thereto, more particularly relates tothe-manufacture of silicon-steel sheets or 5 and plates.

Certain parts alternating magnetic cores, armature poles of various sarily of a laminate .cores,

sheet material of electrical apparatus, having fields, such as transformer pole pieces and magnetic kinds are preferably or necesd structure, being composed of plates or laminations stamped or cut from sheet iron or steel. analogous purposess All sheet metal for such hould possess a highdegree of magnetic permeability, a high degree of electrical resistivity in order to minimize eddy current losses, a low degree of magnetic hysteresis,

and be physically and free from brittleness.

steel, notably that employed in transformer cores strong and as pliable as possible Some sheet iron and and to a lesser extent, that used in alternating current generators, must hysteresis loss. the hysteresis sideration; but it is hysteresis loss with physical properties hereinbefore It is commonknowledge that desirable to-combine low the element silicon in nishes, if properly proces In transformer cores loss is a most important conbe characterized by low especially high permeability and the mentioned. steel containing sufiicient amounts but free as is commercially prac-, other elements except iron, fursed, a sheet which meets all the above described requisites to a remarkable ansformer sheets must also be nonuld not deteriorate with regards to hysteresis losses;

These electrical properties of silicon sheets as to eddy current 1 monly tested in an bined losses being 40 tions. Most manufa therefore, sell their test on an average osses and hysteresis lossesare com- Epstein apparatus, the commeasured at various induc-T cturers of silicon steel sheets, sheets guaranteeing them to below a certain maximum core loss expressed watts per pound or watts per kilogram at a certain frequency as agre and purchaser.

induction and a given ed between the manufacturer For every ingot of'ordinary steel a sheet of certain maid of finish may nature and chem certain minimum "for the sheets rolled mum physical excellenceor quality be obtained. depending on its ical composition. Similarly, a of hysteresis lossis possiblefrom an ingot of silicon steel., The degree of' excellence of the sheets depends considerably-on the treatment which is.

. or grain size allows the crystals to coalesce and fairly sensitive,

It is also well known that when dealing with certain types of low carbon steels the crystals 10 orgrains can be made togrow andabsorb the neighboring grains, thus increasing the crystal in the steel considerably. This crystal growth is brought about by straining the" crystals to a certain point and straining with-an annealing operation which grow. I These large grains'are usually undesirable in most steel where good ductility and considerable strength is desired but-in the case of silicon steels used for electrical apparatus it is generally accepted that large grains tend to lower the hysteresis losses of the steel. The use of silicon inelectrical steel'tends to increase the crystal or grain size of the steel, and this is one of several reasons why in electrical steels.

However, by using the ordinaryrolling and annealing processes, the size; of the crystals or grains obtainable in silicon sheets is somewhat limited. By certain special hot rolling practices it has beenfound' that large grains can be produced after a subsequent annealing operation. However, this variation in hot rolling practice is requires an additional heating and careful manipulation of hot packs.

when successfully manipulated, tends increased grain. size in silicon sheets with attendinglow'hyste'resis losses. These hot rollin p results have been obtained by them. 'I'lie present invention has el necessity of using these special ,hot rolling prac-- tices and provides a process or method 'oftreat- 0 'ing the silicon 7 ly and successfully after they have been hot v rolled without the variations in practice sometimes uriavoidable in handling hotflpacks in hot rolling.

I have found that I can, by means of. my process, induce in a heat treated siliconsheet exactly the degree oi necessary to produce grain growth in the it the attendant superiority as to low'hysteresis following this 15 This s are good and very satisfactory imin'ated the sheet material uniformannealingof silicon sheets and with ":1

, By my inventionI am enabled to produce silicon packs or breakdowns mum hysteresis loss 10 steel of much lower hysteresis loss than is sible by the usual mill con sheets, the improvement due to our invention being quite definite and well marked and amounting to as much sheets produced by the usual practice.

Moreover, my improved process enables me to treat sheets which have been brought to the minipossible under the usual methods of sheet rolling and to eifect in them the marked improvement of hysteresis quality and this in addition to what little improvement is made in the eddy current loss due to reduction of gage.

In the usual methods of manufacturing silicon sheets, the sheets are produced under the usu- .al hot mill practice of hot rolling sheets from bars.

the hot rolled sheetsare then cold rolled toflatten them, they are then pickled and heat treated or box annealed. In practically all the prior art methods of manufacturing silicon sheets the sheets are cold rolled and box annealed although not all sheets are pickled or heat treated. In carrying out my improved process the sili-. con steel bars are first hot rolled. They may-be either breakdowns or finished hot rolled sheets or packs. If in packs, they may or may not be opened. The shearing or not shearing of the packs, breakdowns They then may or may not be given the usual cold rolling pass to smooth out the bent opening corners or to flatten the sheet.

The sheets are then usually pickled if a scalefree surface is desired. This pickling step has little or no effect on the final product; therefore,

the inclusion or elimination from the general process is immaterial.

The next step in this process, either before or after opening and followed by a flattening: cold roll pass if desired, and after pickling sheets when desired, is to normalize the sheets by passing the sheets through a furnace which raises the sheets, to a temperature of from 1500 to 2000 degrees Fahrenheit, more or less, this operation being most commonly done in a reducing atmosphere. I have found the above temperature range most effective but a lower or higher temperature may be used with good results.

The term normalize as commonly used in the steel making arts is used to indicate 'a heat-treatment consisting of heatingthe steel above its so called transformation pointfollowed by cooling rather quickly in air or a reducing atmosphere. Silicon steel does not act the same in the normalizing operation as does ordinary steel. Whether silicon steel is actually normalized by heating as above described is an open question among metallurgists.

In this specification and in the claims the term normalize is used to define a heat treatment at from 1500 to 2000 degrees Fahrenheit, more or less, which serves to relieve the hot rolling strains and prepare the-steel for its subsequent elonga-' tion or stretching. This treatment is preferably carried out in a so-called normalizing furnace but may also be carriedout in otherforms of heat treating furnaces.

The above normalizing step serves to eliminate a large part of the rolling strains and'hardness induced by the hot and cold rolling operations.

However, all of the strains are not relieved by this normalizing step but the strain has-been reduced in the sheets to somewhat below critical strain" (that amount of tension or compression in a sheet or in a bar which will promote maxipractice in processing silias 12 to 15 per cent. over or sheets is immaterialr of strain up to this point are all reduced to a common basis of strain. All of the sheets are therefore put into, such.a physical condition that a smallamount of fairly easily controllable strain can be applied, thus bringingall the individual sheets to a point where they have been critically strained or more, which is necessary before furthertreatment to get the maximum improvement in their hysteresis losses.

The above normalizing step is not to be confused with ordinary annealing. Ordinary annealing produces a grain growth in silicon steel which is somewhat beneficial whereas this normalizing step, although not alone sufiicient to produce the desired electrical properties is very much superior to an ordinary annealing at this stage of-the process.

The normalizing step of this method destroys the hot rolling grain structure in order to permit the following steps to develop a proper grain growth not possible unless the hot rolling grain structure is destroyed.

In order to introduce the strain necessary to promotegrain growth according to the natural laws governing the sciences of metallurgy and metallography the normalized sheets are elongated or stretched preferably in a cold rolling mill. This elongation or stretching produces the critically strained condition necessary for the promotion of uniformly large crystals or grains of silicon steel.

Experience has shown'that an elongation of 1 to 2 per cent. is preferable although I do not wish to limit my invention to this range of elongation since quite satisfactory results have been obtained by more or less elongation. ,It will also be noted that I do not wish to limit myself to the use of cold rolling for producing the elongation since other methods of stretching or elongating the sheets may be employed.

The sheets are preferably stretched or elongated while they are yet warm from the normalizing step and this is especially true when the sheets contain from 2 to 5 per cent. silicon since such sheetsare much more ductile and workable when warm ,and may be elongated with less danger of breakage. The temperature at which the sheets are best suited for elongation orstretching depends largely upon practical working conditions, as for instance, whether the weather is hot or cold, whether the sheets are brittle, the width and gage of the sheets and many other considerations.

The temperature at which the sheets are best suited for stretching or elongation, however is generally from 200 to 400 degrees Fahrenheit for physical reasons before mentioned but I do not wish to limit myself to any specific temperature or range of temperatures as I have found that produce sheets having a minimum hysteresis loss in order to further develop the grain growth and former cores,

and other excellent desirable for transalternating current generators,- reslstance grids, etc.. In obtaining sheets by my method which have equivalent-electrical properand is to be so construed.

ties of sheets produced .reduction in silicon content is permissible, re-- normalizing, since by the prior arts, a

ductile product, since this recontent morethan compensulting in a more duction in silicon sates in ductility for the brittleness accompanying Sheets are therefore more workable when sheared larger grainsize.

or stamped intoshapes. The words sheet or sheet material used throughout this specification and claim are meant to include all 'sheet-likebodies whether'in' sheet or strip form, plate, packs or breakdowns I do not wish to be It will also be noted the.

limited to the exact temperatures at which the sheets are preferably stretched or elongated after this step may .be carried out with good results at any temperature below the criticalor recalescence point.

' "I While I have; described sequence of steps and certain a certain specific specific detailsof the several steps it will be understood that I do not wishto be limited thereto since various'modiicon therein, the amount .terial for electrical purposes material, normalizing the sheet a as-deflned in the appended claim.,

I do not wish to annealing temperature or range of temperature as satisfactory results have been obtained with extreme variations upon the nature'of the hot normalized sheet material while still materially above atmospheric temperature from the normalizing operation to introduce .the critical strain necessary for the promotion of uniformly large crystals or grains, and then annealingv said stretched and elongated sheet material'to induce the maximum'grain growth and improve the electrical qualities thereof.

' ORLAND E. B'OMIG.

in temperature, depending the point of 3 flcationsmay be practiced without departing from the scope of my invention,

limit myself to any specific steel, the percentage of silof strain in thesheets a