US3438820A - Silicon steel process - Google Patents

Description

April 15, 1969 N. P. 6085 ET AL.

SILICON STEEL PROCESS Filed April 2. 1965 INVENTORS NORMAN P. GOSS-JAMES B LACK WlLLI M J. STEWART BY PATENT AGENTS United States Patent Office 3,438,820 Patented Apr. 15, 1969 3,438,820 SILICON STEEL PROCESS Norman P. Goss, Mentor, Ohio, and James Black and William J. Stewart, Burlington, Ontario, Canada, as-

signors to Dominion Foundries and Steel, Limited,

Hamilton, Ontario, Canada Filed Apr. 2, 1965, Ser. No. 445,023 Int. Cl. C21d 7/14, 3/04 US. Cl. 148-110 14 Claims ABSTRACT OF THE DISCLOSURE In a process for the production of gain-oriented silicon steel for magnetic purposes and containing 2.5% to 4% silicon the steel is made by the Basic Oxygen process, but the process is continued until the steel reaches at least about 3000 F. in the vessel and is discharged at at least about 2900" F, to the ladle; the steel is stirred in the ladle and discharged to an ingot mould; the resulting ingot is hot rolled and then cold rolled to obtain the oriented crystal structure; the cold rolled material is unexpectedly magnetically clean, giving improved magnetic properties, and can be decarburized unexpectedly rapidly in a period of not less than about 5 seconds and not more than about one minute to carbon contents of not more than about .005

This invention relates to processes for the production of silicon steel of the type suitable for the production of highly grain-orientated silicon steel for magnetic purposes, and to processes for the production of such highly grain-orientated silicon steel. More particularly, the invention relates to processes for the production of silicon steel suitable for the production of strip and sheet having the desired Goss texture, and in which there is a high degree of [110] (001) orientation (or cube-on-edge crystal orientation) in the direction of rolling, and to processes for the production of such strip and sheet.

There is a constant endeavor on the part of the producers of highly grain-orientated silicon steel strip and sheet to provide more economical and effective processes for its production and/or to improve the magnetic characteristics thereof, the improved characteristics that are sought being higher permeability, lower resistively, and consequent lower core loss.

US. Patent No. 2,867,557, issued Jan. 6, 1959 to J. H. Crede et a-l., states that:

The state of the art and knowledge in the industry have now made it possible to produce silicon steel strip and sheet material having excellent magnetic characteristics with the result that any improvement in the processing which will provide an improvement of as little as 2 or 3% in the watt loss of the resulting magnetic material is nowconsidered as a major contribution to the industry.

It is therefore an indication of the advantages flowing from the processes of this invention, that we are able to provide highly grain-orientated silicon steel strip and sheet with which the core loss is about less than comparable materials commercially available hitherto.

It is known in this particular art that a complete process of making silicon steel and forming it into grainorientated rolled strip suitable for electrical purposes requires careful choice of a large number of parameters, some of which are critical to the economical production of commercially useful material. The effect on the finished material of varying many of these parameters can be determined, but many others are interdependent in a more or less complex manner and their actual effect is the subject of speculation, experiment and argument among those skilled in the art. To better explain our new processes and the advantages obtained thereby, it is believed most convenient first to give an outline of typical prior art processes that have been proposed hitherto.

The silicon steels used commercially for this purpose have a silicon content of about 2.5% to 4%, the higher silicon contents being preferred because of the better magnetic properties that are obtainable. In general the production of material with, a silicon content greater than about 3.3% has not been commercially economical hitherto, because of the extensive edge cracking and breaking encountered during the subsequent cold rolling operations.

The carbon content of the steel must be kept as low as possible and to this end it has either been produced by an open hearth process using oxygen, for example a process as described by US. Patent No. 2,580,164 issued Jan. 1, 1952 to Slottman, or by processes known in the industry as Basic Oxygen steelmaking processes, particularly that known as the LD Process, as described in U.S. Patent No. 2,800,621, issued July 23, 1957 to Suess et a1. Problems encountered hitherto in obtaining low carbon contents with an LD process are described in U. S. Patent No. 3,030,203, issued Apr. 17, 1962 to Hilliard, which describes a special oxygen-blowing procedure intended to duplicate open hearth practice and maintain a steel bath at a temperature of 2900 F., plus or minus 30 F., thereby producing a steel with carbon content of not more than 0.035%, without the entrapment of slag or pickup of nitrogen.

Silicon and sulphur are added to the steel at the end of the steelmaking process, and may be added in the furnace, or in the ladle into which the steel is poured from the furnace; more usally the latter. The purpose of the sulphur is to form manganese sulphide with the maganese always present; this particular sulphide has the property of entering the grain boundaries as the steel cools and crystallizes and ensuring the formation of the desired primary grain structure during the hot rolling operation.

The silicon is usually added to the steel in the form of ferrosilicon which dissolves readily to form layers of silicon content higher than is desired, and these higher-silicon-content layers tend to remain separate from the lower-silicon-content layers unless positive action is taken to prevent this so-called Stratification. To this end it has been a known practice to pour the contents of the first ladle into a second ladle, and frequently into a third ladle, thus ensuring thorough mixing before the molten steel is finally poured into ingot moulds. For the same purpose it is proposed in the above-mentioned Hilliard patent to rock the furnace.

The ingots thus produced are usually heat soaked for a predetermined period and then rolled to slab form, each sla'b thereafter being hot rolled to strip form, either by the so-called direct hot working process or by the slab-reheating process. In the first mentioned process the slab is rolled without any intermediate reheating, while in the second process it is reheated to a high temperature in the neighbourhood of 2300 F. to 2550 F. before being rolled into strip. The hot rolling to strip form is a critical step in the formation of commercially useful material because of the formation of the abovedescribed primary grain structure and relatively precise control of the rolling temperatures has been regarded as essential. For example, US. Patent No. 2,599,340, issued June 3, 1952 to Littmann et al., describes a process in which the slabs are annealed at about 2500" F. and rolled at about 2300 F. The above-mentioned Crede et al. patent describes a process of heating the ingot to about 2300 'F., producing a hot slab of temperature not less than 2000 F., and immediately reducing the slab to strip before its temperature decreases to below 3 1600 F. The usual range of hot rolling temperatures used hitherto has been between 1600- F. and 2300 F., and an unexplained gap has existed between 1450' F. and 1600 F, the material obtained by rolling at temperatures in the gap being of little or no commercial value.

The hot rolled strip is descaled and is cooled quickly to prevent further grain growth and keep the primary grains small and highly dispersed. The rapid cooling also prevents conversion of the finely dispersed carbon inclusions into carbides that can only be removed with considerable dilficulty. The cooled strip is then given a first cold roll to a thickness usually of about 50% of the desired final thickness, the degree of reduction employed being carefully selected because of its effect on the desired orientation of the grain structures in the direction of rolling. It has also been essential, once a particular intermediate thickness has been selected, to maintain that thickness to within very close limits. The cold rolled strip is cleaned and open annealed, usually at about 1650-1750 F. to provide stress relief and cause recrystallisation.

The partially reduced strip is then cold rolled to its final thickness, which usually is about .006 inch to .014 inch, the degree of reduction again being carefully selected and the rolling limits carefully controlled for the same reason. The rolled strip is subjected to a decarburization treatment intended to reduce the carbon content to the lowest commercially practical value. For example, U.S. Patent No. 2,287,467, issued June 23, 1942 to Carpenter et al., describes a process in which the decarburizing conditions are intended to achieve a carbon content of about 0.005% and comprise a temperature within the range 1350 to 1650" F., and an atmosphere predominantly of hydrogen, but containing 4% to 35% water vapour; preferably the duration of the Carpenter et al. process is not to exceed 30 minutes, and more particularly it is take between 2 and '14 minutes.

In the final stages of the production processes the strip is coated with a refractory oxide and subjected to a desulphurizing box anneal at about 2000-2200 F. in an atmosphere of pure hydrogen, this process causing the desired exaggerated secondary grain growth of the orientated cube-on-edge primary grains. Thereafter, the oxide coating is removed, the strip is thermally flattened if necessary, and the strip is cut to its final strip or sheet form. t

For commercial purposes a premium quality magnetic steel preferably has an A.C. permeability at a magnetizing force of oersteds of not less than about 1800,

and .in the above-mentioned Crede Patent No. 2,867,557 a permeability of between 1787 and 17-94 is considered remarkable for commercial material. Test reported in the Westinghouse Engineer for September 1952, and made on single crystals of steel containing approximately 3.0% silicon indicated a possible theoretical maximum permeability of about 2015, the actual value of 1940 that was obtained being attributed to the presence of slight impurities in the steel. U.S. Patent No. 2,599,340 issued June 3, 1952 to Littmann et al., states that the maximum possible permeability, i.e. the permeability of a single crystal measured in the best direction, is believed to lie between 1900 and 1950 for steel of 3.2% silicon content, '(but) these values are not attained in practice. It is claimed that in processes including the steps of the Littmann et al. invention permeabilities up to 1835 have been obtained consistently.

It is an object of the present invention to provide a new process for the production of silicon steels suitable for the production of highly grain-orientated silicon steel for magnetic purposes.

It is another object to provide a new process for the production of highly grain-orientated silicon steel strip or sheet suitable for magnetic purposes.

In accordance with the present invention, there is provided a process for the production of silicon steel containing substantially 2.5% to 4% silicon and suitable for the production of highly grain-oriented silicon steel for magnetic purposes, the process comprising the steps of making the steel by a process in which gaseous oxygen is directed into the vessel interior to react with the contents of the vessel, the steel making process being continued until the contents of the vessel are at a temperature of at least about 3000" F., and discharging the resultant steel from the vessel at a temperature of at least about 2900 F.

Preferably the steel making process is continued until the contents of the vessel are at a temperature of at least about 3100" F. to 3250 F.

The steel may be stirred in the said ladle to ensure the absence of Stratification of the contents thereof, and preferably it is stirred with an ingot of substantially the same composition as the said steel. The steel preferably is discharged from the ladle at a temperature of at least about 2850 F. directly into ingot moulds. The ingots thus produced are rolled into slabs which may be hot rolled with an exit temperature of anywhere between 1400 F. and 1700 F. The hot rolled strip is thereafter cold rolled and annealed to produce a strip having the desired grainorientation in the direction of rolling, and subsequently decarburized, preferably by subjecting it for not less than about 5 seconds and not more than about 1 minute to decarburizing conditions such as to reduce the carbon content of the cold rolled strip to not more than about 005%, preferably 004%. Preferably, the said decarburizing conditions comprise a temperature of between about 1350 F. and about 1800" F., and an atmosphere comprising between about 5% and about 40% hydrogen, preferably 20%, remainder nitrogen, with a dew point of about F. and above. More preferably, the said decarburizing conditions comprise a temperature of between about 1650 F. and 1800 F., the cold rolled strip being subjected to such conditions for a period of not less than about 5 seconds and not more than about 20 seconds. Subsequently, the decarburized cold rolled strip may be subjected to a desulphurizin'g box anneal in pure hydrogen at between about 2000 F. and 2200 F. to provide the necessary secondary grain growth and provide a highly grainorientated magnetically clean silicon steel having an A.C. magnetic permeability in a field of 10 oersteds of at least 1900. Preferably, the said magnetically clean silicon steel comprises from 3.3% to 3.7% silicon.

Particular preferred processes embodying our invention will now be described, by way of example, with reference to the accompanying drawing in respect of a particular feature thereof, the single figure of the drawing illustrating a steelmaking furnace, a ladle, and stirring means for the ladle contents.

In the practice of our invention the steel is produced by a process that comes within the terms of the A.S.T.M. definition of a Basic Oxygen steelmaking process, namely one in which molten iron is refined to steel under a basic slag in a cylindrical furnace lined with basic refractories, by directing a jet of high purity gaseous oxygen onto the surface of the hot metal bath. In particular, the process may be defined as an LD process, in which a vertically extending vessel has a lance extending vertically into the vessel, through which lance the jet of oxygen is directed into the vessel interior to react with the contents thereof. The charge to the furnace is convenional for the production of silicon steel and the oxygen blowing is begun with the lance in an elevated position. During the steelmaking process, which usually takes 20 to 30 minutes, the height of the lance above the furnace contents and the rate of blowing of the oxygen are adjusted by the operator, based upon a predetermined schedule for the particular steel to be produced, and the observation by the operator of the progress of the process, usually comprising visual observation of the furnace and measurement of the temperature of the steel bath from time to time.

Also in the practice of our invention the steel making process is controlled by the operator so as to achieve a bath temperature near to the end of the process of at least about 3000 F. Preferably, the operator controls the process so that a temperature greater than 3050" F. is obtained, and more particularly it is preferred to obtain a temperature of between about 3l0O F. and 3250 F. At the present time the upper temperature limit to be achieved by the process appears to be at least partly determined by the ability of the conventional LD furnace to operate safely at these unusually elevated temperatures. The upper limit is also affected by commercial economic considerations in that at the elevated temperatures there is a higher conversion of the iron in the furnace charge to iron oxide that is retained in the slag, with a consequent loss of iron. Consideration must also be given to the ad vantages obtained with these higher hath temperatures that there is a higher conversion of the carbon by the oxygen, with a consequent reduction of the proportion of carbon in the resultant steel. The Table 1 below gives a desirable range and a typical value for the chemical analysis of the steel and slag resulting from a process in accordance with this invention.

TAB LE 1 Furnace steel analysis Range Typical 020-. 030 021 010-. 015 012 09-. 130 003-. 008 005 03-. 10 08 Balance Balance Slag analysis Constituent: FeO, percent- 40-45 44 We believe at present that the unexpected advantages of the processes of this invention arise, at least in part, from the increased fluidity of the liquid steel and the increased fluidity with high surface tension of the overlying slag at these elevated temperatures, so that the magnetically undesirable inclusions have a substantially greater tendency to pass from the liquid steel into the slag layer, and once in the slag layer are more permanently retained therein. It is these inclusions which in the subsequent grain orientation steps restrict the movement of the domain walls to reduce the permeability of the grain orientated material below the value of which it is inherently capable. The processes of the invention produce a steel which is appreciably magnetically cleaner than those produced commercially hitherto at lower temperatures, so that higher permeabilities can eventually be achieved with higher resistivity and lower core loss.

At the conclusion of the steel making process as much as possible of the molten slag is tapped from the furnace vessel, and thereafter the steel is discharged into a preheated ladle through a tap hole below the upper edge of the vessel, the tapping procedure being such as to avoid as far as possible the return of slag particles to the molten steel.

In normal operation of the furnace process there is a terminal stage after the maximum temperature had been reached, during which terminal stage the bath temperature decreases relatively rapidly, so that the steel is discharged into the ladle at a temperature lower than the maximum. This discharge temperature should be at least 2900 F. and preferably is as high as possible, as close as possible to the maximum temperature achieved during the conversion process.

The silicon and sulphur can be added to the furnace, or to the ladle, and if to the latter will be added before the ladle is full, so that the remainder of the entering steel will stir the additions into the ladle contents. To ensure the absence of the above-described deleterious Stratification of the molten steel, the contents of the ladle are stirred before the ingots are poured, using an already-formed ingot of substantially the same composition as the steel. In the drawing accompanying this application there is illustrated a vertical-extending LD process furnace vessel 1 having a lance 2 through which a jet of oxygen is directed into the vessel onto the surface of the hot metal bath. As illustrated in the drawing, steel has been discharged from the vessel 1 through a tap hole 3 into a ladle 4. The contents 5 of the ladle are in the process of being stirred by means of an ingot 6 which is suspended by a chain 7 from an overhead crane 8 with its lower portion in the molten metal. By operation of the crane controls the operator thereof can move the ingot in the ladle with a circular backand-forth stirring motion that will in about 2 to 5 minutes of operation ensure adequate homogeneity of the whole of the ladles contents.

This simple and rapid stirring process in a single ladle may be contrasted with the prior art processes of transfer between ladles, with its attendant appreciable cooling of the molten steel, and the high probability of pick-up of undesirable materials during the transfer operations.

The stirred homogeneous steel is thereafter discharged from the ladle into preheated ingot moulds, the steel being discharged from the ladle at a temperature of at least about 2850 F. and preferably in the range 2870 F. to 2900 F., so that the ingots will be poured at a temperature of not less than about 2840 F., and preferably higher. It is known in prior art processes to maintain the ladle contents at as high a temperature as possible by the addition of slag thereto, but such a method is avoided in the practice of this invention because of the possibility of returning deleterious slag inclusions to the relatively magnetically clean steel.

At the high temperature of the ladle contents a part of the ingot is dissolved away into the steel during each stirring operation, and the ingot must usually be discarded after about 8-12. operations. It is at present believed that the use of an ingot for stirring at these elevated temperatures may have an advantageous side effect due to the scale with which the ingot is coated before the stirring operation begins. This scale becomes dispersed in the steel and thereafter rises to the surface slag, and in the high-temperature, high-fluidity steel this dissolved scale may be unusually effective in removing magnetically deleterious material from the steel. It should be noted that in cooling in the normal ambient atmosphere after use the ingot may cool sufficiently to have moisture deposit thereon, and it should be sufficiently heated before it is again used to remove this moisture, since otherwise it will add undesired hydrogen to the steel.

The Table 2 below gives a desirable range and a typical value for the analysis of the steel discharged from the ladle.

TABLE 2.LA.DLE STEEL ANALYSIS Element Range Typical 3. 00-3. 6 3. 5 020-. 030 022 010-. 030 015 1175-. 010 009 004-. 008 006 Balance Balance At the low oxygen contents obtained magnetically undesirable non-metallic material in the bath can more readily form endogenous inclusions which will separate from the highly fluid steel and enter and be trapped in the slag. The removal of these inclusions is also assisted by assuring that the deoxidization products produced in the ladle are such that the slag or scum developed therefrom has a high fluidity and high surface tension at the elevated temperature of opreation, causing the desired rapid coalescence of slag particles in the melt to more readily separated particles and the high retention of r these particles in the slag or scum.

The cast ingots are reheated prior to the hot rolling operation to about 2250 F. to 2350 F., and are held at this temperature for a period sufficient to ensure homogenisation of the microsegregation of the various elements of the steel. Each ingot is first reduced in a hot rolling mill to an intermediate thickness of about As-P/z inches, usually A; inch. The temperature at which this rolling is effected should be as high as possible, and the rolling should be completed as quickly as possible, but we have found that with a silicon steel in accordance with this invention a much lower temperature is permissible. In particular with our improved practice the temperature just before entering the finishing stands of the mill can be as low as 1800 F.

The hot rolled intermediate thickness material is then subjected to a final hot rolling in which its thickness is reduced to .060 to .120 inch, usually about 070-080 inch. As described above, temperature control during hot rolling is essential in that it causes the development of the primary grain structure which will be developed into the described secondary grain structure during the desulphurizing treatment. We have now found that satisfac-tory materials are produced if the final hot rolling is carried out with exit temperatures anywhere between about 1400 F. and 1700 F., and in particular the temperature gap found in the prior art processes no longer appears to exist, or if it does exist its effect on the production of commercial materials need not be considered. The practical commercial advantages of this widening and non-criticality of the exit temperature range available for hot rolling will be apparent to those skilled in the art, and, for example, the exit temperature to be employed can be determined by consideration of other factors, such as the proper loading of the rolling mill and the surface finish required on the rolled Strip.

Experience in rolling the materials produced in accordance with the present invention show that for a given silicon content they are unexpectedly more ductile than the known commercial materials, so that materials of high silicon content e.g. 3.3 to 3.7% can be hot rolled successfully without the occurrence of an uneconomic amount of edge cracking and/or coil breakage.

Following the hot rolling, conventional processes and apparatus may be employed for descaling and for cooling the strip quickly to below about 1000 F. Conventional practices may also be followed in developing the orientated grain structure by cold rolling the descaled strip to a thickness of about 50% of the desired final thickness, cleaning the cold rolled strip, stress relieving and recrystallising the strip by open annealing at about 15001700 F., and thereafter cold rolling the strip to the final desired thickness, which will usually be .012 inch. In these operations also an unexpected advantage is obtained due to the increased ductility of the magnetically clean material, this increased ductility permitting higher silicon content materials to be cold rolled successfully without uneconomic cracking or breaking, or permitting materials of the same silicon content to be cold rolled at lower temperatures, e.g. at 120 F. or even down to ambient temperatures greater than 70 F. for material of 3.5% silicon, instead of 170 F. required hitherto for material of 3.4% silicon or less.

The finally rolled strip must now be subjected to a decarburization treatment, and it is with such treatment that a still further unexpected advantage of the invention is obtained. Thus, we have found that a decarburization treatment to obtain about 005% carbon, which with prior art processes required at least 2-4 minutes for completion, can with a process of our invention be effected in less than one minute, and, in particular, less than 30 seconds, and under certain conditions the decarburization can be effected in about 5 to seconds. Referring again to the above-mentioned patent of Carpenter et al., they propose a range of 1350 F. to 1650 F. for the decarburization treatment, and found that above 1650 F.

they were unable to achieve the desired low carbon content, their process exhibiting a minimum at about 1500 F. On the contrary we have found the higher temperatures to be advantageous in reducing the carburization time by a factor of about 10-20, and are able to make use of temperatures as high as 1700 F. with maximum reduction of carbon cotnent.

The Table 3 below shows the effect of the annealing temperature and the soak time of the decarburizing atmosphere on the carbon content of steels in accordance with this invention. The decarburizing atmosphere employed was 20% hydrogen, nitrogen with water vapour to give dew point of 6070 F.

In a process in accordance with this invention the preferred decarburizing conditions comprise a temperature of between about 1350 F. and about 1800 F., using an atmosphere comprising between about 5% and about 40% of hydrogen, the balance nitrogen and water vapour, with a dew point of about 60 F. and above. The cold rolled strip is subjected to such conditions for a period of not less than about 5 seconds and not more than a period of one minute, preferably not more than 30 seconds and generally not more than 15 seconds. The faster decarburization times will of course be accomplished at the higher temperatures, and we have found that the carbon removal process has commenced at temperatures at low as 1300 F. The dwell time of the strip in the annealing furnace will usually be greater than the actual decarburizing time, since the strip enters the furnace at ambient atmospheric temperature and will take time to reach the minimum reaction temperature.

It is at present believed that these unexpectedly brief decarburization times are due at least in part to the high diffusion coefiicient obtained with the unusually magnetically clean steel that has been obtained. It is also found that the decarburization appears to begin at a lower temperature than has been common in prior art processes. The lower carbon contents of the steels produced in the furnace and the ladle will of course assist in this effect in that there is less carbon to be removed.

The decarburized strip may then be subjected to conventional processes of coating with an inorganic refactory material such as magnesium oxide, and desulphurization by a box anneal in pure hydrogen at about 2000 F.2200 F., preferably about 2050 F. Thereafter, the refactory material may be removed, the strip thermally flattened if necessary, and a suitable protective coating applied.

The following Table 4 sets out the general chemical ananlysis of highly grain-orientated silicon steel strip material produced in accordance with the present invention, together with a typical analysis of a particular material.

TABLE 4.-STRIP CHEMICAL ANALYSIS Element Range Typical Balance Balance 1 Maximum.

The following Table 5 sets out the general range and a typical magnetic analysis of a prior art highly grain- TABLE 5.-STRIP MAGNETIC ANALYSIS Range Typical Range Typical prior art prior art this this ininvention vcntion Average permeability at H 1, 761-1, 869 1, 809 1, 900-2, 000 1, 950 Core loss, Watts at 15 kg.

and 60 cycles 525-. 592 553 .4601 520 50 What is claimed is:

1. In a process for the production of silicon steel con taining substantially 2.5% to 4% silicon and the subsequent production therefrom of highly grain-orientated silicon steel for magnetic purposes, the steps of making steel by a process in which gaseous oxygen is blown into the vessel interior to react with the contents of the vessel, the steel making process being continued until the contents of the vessel are at a temperature of at least about 3000 F., discharging the resultant steel from the vessel at a temperature of at least about 2900 F., forming at least one ingot from the said steel, hot rolling each ingot to produce a strip having a primary grain texture, cold rolling and annealing the said strip to a smaller thickness to develop an orientated grain texture in the direction of rolling, and decarburizing the cold rolled strip by subjecting it for not less than about 5 seconds and not more than about one minute to decarburizing conditions such as to reduce the carbon content of the cold rolled strip to not more than about .005

2. A process as claimed in claim 1, wherein the steel working process is continued until the contents of the vessel are at a tempenature of at least about 3100 F. to 3250" F.

3. A process as claimed in claim 1, wherein the steel is discharged from the said vessel to a ladle, and subsequently the steel is discharged from the said ladle directly to at least one ingot mould to form said at least one ingot at a temperature of at least about 2850 F.

4. A process as claimed in claim 1, wherein the hot rolling is eifected to a thickness of between about .060 to about .120 inch with an exit temperature of between about 1400 F. and about 1700 F.

5. A process as claimed in claim 1, wherein the said decarburizing conditions comprise a temperature of between about 1350 F. and about 1800 F., and an atmosphere comprising about between 5% and 40% hydrogen, balance nitrogen and water vapour with a dew point of about 60 F. and above.

6. A process as claimed in claim 1, wherein the cold rolled strip is subjected to the said decarburizing conditions for not more than about 30 seconds.

7. A process as claimed in claim 3, wherein the said decarburizing conditions comprise a temperature of between about 1650 F. and 1800 F. and an atomsphere comprising between about 5% and about 40% hydrogen, the balance nitrogen and water vapour with a dew point of about 60 F. and above, and the cold rolled strip is subjected to such conditions for a period of not less than about 5 seconds and not more than about 30 seconds.

8. A process as claimed in claim 3, and including the step of stirring the steel in the ladle to ensure the absence of Stratification of the contents thereof.

9. A process as claimed in claim 4, wherein the said cold rolling is carried out at temperatures between about 70 F. and 120 F.

10. A process as claimed in claim 5, wherein the said 10 decarburizing conditions comprise a temperature of between about 1650 F. and about 1800 F. and the cold rolled strip is subjected to such conditions for a period of not less than about 5 seconds and not more than about 30 seconds.

11. In a process for the production of magnetically clean silicon steel containing substantially 2.9% to 3.7% silicon and the subsequent production therefrom of highly grain orientated silicon steel for magnetic purposes and having an AC. magnetic permeability to a field of 10 oersteds of at least 1900, the steps of making the steel by a process in which gaseous oxygen is. blown into the vessel interior to react with the contents of the vessel, the steel making process being continued until the contents of the vessel are at a temperature of about 3000 F., discharging the resultant steel from the vessel at a temperature of at least about 2900 F., forming at least one ingot from the said steel, hot rolling each ingot with a final temperature between about 1400 F. and about 1700 F. to produce a strip having a desired primarly grain texture, cold rolling an annealing the said strip to a smaller thickness to develop a grain structure orientated in the direction of rolling, decarburizing the cold rolled strip to reduce the carbon content thereof to not more than about 005%, by subjecting it for not less than about 5 seconds and not more than about 20 seconds to a temperature of between about 1650 F. and 1800 F. and an atmosphere comprising between about 5% and about 40% hydrogen, the balance nitrogen and water vapour with a dew point of about 60 F. and above, and subjecting the decarburized cold rolled strip to a desulphurizing box anneal in pure hydrogen at between about 2000 F. and 2200 F. to provide secondary grain growth.

12. A process as claimed in claim 11, wherein the steel working process is continued until the contents of the vessel are at a temperature of at least about 3100 F. to 3250 F.

13. A process as claimed in claim 11, wherein the said highly grain orientated silicon steel produced from the said magnetically clean silicon steel comprises from 3.3% to 3.6% of silicon.

14. A process as claimed in claim 12, wherein the steel is discharged from the said vessel to a ladle, the steel in the said ladle is stirred to ensure the absence of stratification of the contents thereof, and subsequently the steel is discharged from the said ladle directly to at least one ingot mould to form the said at least one ingot at a temperature of at least about 2850 F.

References Cited UNITED STATES PATENTS 2,287,467 6/ 1942 Carpenter. 2,529,373 11/1950 Campbell 1481 10 2,599,340 6/1952 Littmann. 2,867,557 1/1959 Crede. 3,021,237 2/1962 Henke 1481 13 XR 3,030,203 4/ 1962 Hilliard. 3,039,902 6/1962 Miller 1481l3 XR 3,196,054 7/1965 Carpenter 148110 XR FOREIGN PATENTS 845,167 8/1958 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

US. Cl. X.R.

Images