US3502446A - Production of powder,strip and other metal products from refined molten metal - Google Patents

Description

M. D. AYERS March 24, 1970 3,502,446 STRIP AND OTHER METAL PRODUCTS FROM REFINED MOLTEN METAL PRODUCT ION OF POWDER C5 Sheets-Sheet 2 Original Filed Oct. 8, 1964 INVENTOR. MAURICE D. AYES ATTORNEYS 3,502,446 STRIP AND OTHER METAL PRODUCTS March 24, 1970 M. D. Avr-:Rs

PRODUCTION OF POWDER,

FROM REFINED MOLTEN METAL Original Filed Oct. 8, 1964 5 Sheets-Sheet 3 INVENTOR. MAURICE D. AYE 39 Z BY y ATTORNEYS United States Patent O 3,502,446 PRODUCTION OF POWDER, STRIP AND OTHER METAL PRODUCTS FROM REFINED MOLTEN METAL Maurice D. Ayers, Stamford, Conn., assignor to Metal Innovations, Inc., Stamford, Conn., a corporation of Delaware Application Oct. 8, 1964, Ser. No. 402,480, now Patent No. 3,334,408, which is a continuation-impart of application Ser. No. 321,246, Nov. 4, 1963. Divided and this application Aug. 7, 1967, Ser. No. 658,837

Int. Cl. B221? 3/00, 5/00, 7/00 U.S. Cl. 29-182.5 4 Claims ABSTRACT OF 'IHE DISCLOSURE The invention is directed to the production of iron strip from atomized iron, and particularly to the production of an iron strip having superior electrical and mechanical characteristics for use in electrical applications, such as small motors, solenoids, transformers, etc.

RELATED APPLICATION This application is a division of my copending application Ser. No. 402,480, filed Oct. 8, 1964, now U.S. Patent No. 3,334,408. The copending application, in turn, is a continuation-in-part of my earlier application Ser. No. 321,246, filed Nov. 4, 1963, now U.S. Patent No. 3,281,- 893.

BACKGROUND AND PRIOR ART In the production of merchant products and other substantially finished forms of steel, it is conventional to cast the refined or controlled analysis molten metal into large ingots. These ingots are then transferred to large soaking pits where they are kept for a period of time and are brought to a uniformly high temperature. After a desired soaking period the heated ingots are transferred to a slabbing or blooming mill and rolled into slabs, blooms, or billets. These intermediate products often are then inventoried and subsequently reheated for hot rolling and sometimes also cold rolling into strip, bars, or rods, in a sequence of operations usually involving special heating cycles, pickling, annealing, and the like. The output of the secondary rolling procedures desirably is in a form suitable for delivery to the steel consumer.

In more recent developments having limited acceptance in the steel industry, the molten metal is continuously cast into slabs or billets, after degassing, which has an advantage of eliminating ingot pouring, soaking pits, and slabbing or blooming mills, but still involves all of the procedural steps and equipment of the merchant or finishing mill, for converting the slabs and billets into final end products.

In either the conventional processing or in the `continuous casting system, each step of the overall process is of a character to require huge installations of plant and equipment and large capital investments. For example, economical inst-allations for strip production typically must have a capacity of at least 300,000 tons annually (and usually upwards of 500,000 tons), and the capital and other requirements of such installations tend to limit participation in the industry to a relatively few well-financed companies at a relatively limited number of geographical locations.

Significant economic and other advantages are realized through a novel process, covered by my Patent No. 3,334,- 408, of converting molten metal directly into the form of strip, bars, or rods, in an effectively uninterrupted process involving Ian intermediate conversion of molten steel to iron powder and the subsequent conversion of the pow- 3,502,446 Patented Mar. 24, 1970 ACC dered iron or steel to the desired strip or other finished or semi-finished form. In following the new simplified process, significant economies are realized and substantial reductions in capital costs for equipment are made possible.

In addition to the above-stated and other advantages of an essentially economic nature, the process of my Patent No. 3,334,408 affords a wide range of flexibility in the types and quality of the products capable of being. produced, through control over the molten metal primary input and through blending and other operations which are possible after the primary molten metal component has been converted to its intermediate, powder stage. This is of particular importance in that it enables the metal product to be specifically tailored to the desired end use, rather than accommodating the end use to the available metals as has been more common heretofore. Improved product quality is realized not only through close control of the metal refining and analysis adjustment, made possible by the integrated nature of the process, ibut also by the avoidance of defects otherwise arising through the conventional ingot casting, rolling and processing.

Generally stated, the process of the invention involves the preparation of a desired analysis molten iron or steel. The molten metal is transferred directly to an atomizing chamber, in which one or more streams of the metal are intercepted by high-pressure jets of liquid, usually water, and the molten metal is converted to a desired powder form.

After refining of the molten steel or iron and conversion of it into desired high purity powdered metal, the powder, entrained in its cooling Water, is fed to a separator, which removes the majority of the Water constituent. The dewatered but still wet powder is then dried and screened with respect to particle size, and the dried powder is transferred to suitable holding bins or hoppers from which it is controllably fed into the strip forming stage of the process.

The powder is drawn from the storage bins in a precisely controlled manner and, where desired, is blended with appropriate alloying powders and/ or addtives. The powder or blended powder may be then fed through =a preheating zone, which heats the powder to a temperature at which the powder tends to become soft and plastic without, however, becoming too stickly to process. Thereupon the heated powder is directed with controlled rate and distribution between a first stage of compacting rollers which compress the powder into a so-called green strip which is self-supporting, although Weak, and has a density of about 70 to 95 percent.

The partially compacted green strip is directed into a special heating chamber, in which the green strip iS brought up to a higher temperature, sufficient to enable a second stage of compacting to be carried out, to reduce the strip to substantially percent density. In addition, the heating chamber represents an ideal place for subjecting the metal to various reactive treatments, since the metal is still in a highly porous form. Such reactions as carburizing, decarburizing, deoxidation, nitriding, lchromanizing, nickelizing, etc., may be carried out with high efficiency, because of the porous nature and high area exposure of the partially compacted metal.

After compacting to substantially 100 percent density, the strip has substantially conventional characteritsics. Because it is at an elevated temperature at this point, and it is still subject to the protection of the controlled ambient, the strip is additionally hot-rolled in one or more stages to a desired gauge, which typically could be well below the conventional hot-rolled gauges, because the starting strip thickness is considerably less than in the case of conventional hot-rolling procedures.

After cooling to reduce the likelihood of oxidation, the strip is brought out into the open atmosphere, subjected to such optional treatments as may be appropriate, such as cold-rolling to impart desired surface characteristics or temper, and then sheared or coiled, as desired.

SUMMARY OF INVENTION One of the particularly advantageous specific end products which may be produced in accordance with the invention is iron strip of the type used in the manufacture of motors, transformers, and the like. Iron strip produced in accordance with the invention can be controlled to have particularly desirable characteristics for electrical applications and yet be produced at a cost which is significantly below the cost of conventional electrical strip. Electrical strip must, of course, have desirable magnetic properties in addition to being relatively fiat, suitable for high speed punching and shearing operations, and otherwise suitable for fabrication into laminated, magnetic structures providing limited current losses. One of the most significant factors affecting magnetic properties of the strip is the content of carbon and nitrogen impurities. These impurities must be reduced to a low level, which has occasioned substantial expense pursuant to known practices, but can be readily and economically controlled to the desired low levels according to the procedures of the invention. The nature and size of the grain also has an important effect upon magnetic properties, as well as upon mechanical properties such as punchability, resistance to cold welding, softness, etc., desired in an electrical strip, and these properties are optimized by the procedures of the invention. Mechanical stresses and grain refinement, typically introduced during conventional rolling operations and known to adversely affect the magnetic properties of electrical strip, are signicantly reduced in following the procedures of the invention, because the initial thickness of the strip, as produced directly from powder, is relatively small and the number and character of subsequent rolling operations is significantly reduced as compared to more conventional procedures. For example, strip made in accordance with the invention has been compacted at 0.040 inch thickness and processed to finished strip 0.025 inch thick. This strip had excellent electrical characteristics.

For a more complete understanding of the invention, reference should be made to the following detailed description and to the accompanying drawing.

DESCRIPTION OF DRAWING FIG. la and FIG. 1b together constitute a greatly simplified, schematic representation of a process, according to the invention of my Patent No. 3,334,408 for the direct and continuous conversion of molten metal to substantially finished products, such as strip, bars, and rods.

FIG. 2 is a fragmentary cross-sectional view of a moditied and advantageous form of atomizing chamber for making metal powders.

FIG. 3 is a fragmentary cross-sectional view taken gerr erally along line 3 3 of FIG. 2.

FIG. 4 is a fragmentary cross-sectional view illustrating an improved arrangement, according to one aspect of the invention, for feeding preheated metal powder into a set of compacting rolls.

FIG. 5 is a fragmentary cross-sectional View taken generally along line 5-5 of FIG. 4.

DESCRIPTION OF INVENTION Referring now to the drawing, the reference numeral designates a body of molten metal, which is being refined or adjusted as to analysis in a suitable vessel 11. The vessel 11 may be any suitable facility for treating a molten metal body 10, and typically the vessel will be an open hearth furnace, an electric furnace, an L-D convertor, or the like suitable for rening steel. In the processing of steel, to which this invention is particularly directed, the vessel 11 advantageously will perform a refining function, to produce a molten iron of the highest practicable purity,

4 even though it may be necessary or desirable, later in the process, to reintroduces carbon or other alloying agents.

At appropriate times, the refined and/or controlled analysis molten metal 10 is discharged from the vessel 11, typically into a suitable ladle 12, lby means of which the molten metal is conveyed to and controllably discharged into an atomizing vessel designated generally by the reference numeral 13. In the specifically illustrated system, the vessel 13 includes an upper housing section, forming an atomizing chamber 15, and a lower housing section 16, forming a collection or receiving chamber. In the top wall of the upper housing section is mounted a receiving Crucible 17, provided in its bottom area with one or more (advantageously a plurality) discharge openings 18 for directing a plurality of small, discrete streams of the molten metal into the atomizing chamber 15. Desirably, the openings 18 are of about 1A inch diameter, and a diameter in excess of about 1/2 inch probably would be too large for an atomizing chamber of typical proportions. A pair of water discharge nozzles 19 are disposed in suitable array within the atomizing chamber and are arranged to direct high pressure (e.g., upwards of 400 p.s.i.) streams of atomizing water inward and downward, toward and into intercepting relation to the metal stream discharged from the receiving crucible 17.

Advantageously, the relationship of the molten metal stream to the atomizing water jets is such that the jets forcibly disperse and quickly quench the molten metal and thereby produce predominantly sharp and irregular powder particles, rather than spherical particles. In this respect, because of surface tensions and other influencing factors, the atomized molten metal has a tendency to form substantially spherical powder particles which are less desirable for subsequent compacting into metal products because of the inability of spherical particles to pack closely and to interlock with adjacent particles. The more desirable irregular particles may be achieved in accordance with the invention by utilizing advantageous forms of quenching streams, issued at sufficiently high water jet velocity, and discharging the molten metal from the receiving crucible in suliiciently small individual streams.

The atomized powder particles are produced in predominantly irregular form capable of eicient interlocking by so directing the water and metal streams as to prevent substantial contact between the issuing metal streams or metal droplets and small droplets or bubbles of water. This is accomplished by so designing and constructing the water discharge nozzles 19 as to cause them to issue streams of quenching water in the form of solid sheets, rather than in the form of sprays comprised of many individual streams or droplets. To this end, it is advantageous to utilize a nozzle having an elongated discharge slot of, for example, about 1/2 inch in length and 1/32 inch in thickness. The nozzle 19, as shown in more detail in FIG. 3, is advantageously adapted, when operated under appropriately high pressure (typically about 500 p.s.i.) to issue a fan-shaped solid sheet of water, as indicated at 70 in FIG. 3. In an advantageous installation, the solid sheet of water will fan out to a width of about 6 inches and a thickness of about 1A; inch, at a distance of about 12 inches from the nozzle.

A cooperating pair or pairs of water discharge nozzles 19 advantageously are so arranged that their downwardly and inwardly directed sheet-like streams of water intersect to form a V-shaped trough, with the apex of the trough being located directly below the molten metal discharge opening 18, such that the discharging streams or droplets of molten metal drop generally symmetrically into the trough.

At the point at which the water streams 70 converge and intercept the descending body of molten metal, there is a substantial tendency for the water to bubble and froth, due in some degree simply to the force of the converging streams and in some degree to the effect of the molten metal being intercepted by the water. It has been found that the formation of bubbles and froth at this point is significantly detrimental to the formation of proper powder particles, because of undesirable steam generation and for other reasons. Thus, the atomizing installation of the invention incorporates, in addition to nozzles adapted to issue solid sheets of quenching water, an arrangement in which the nozzles are disposed at a sufficiently small angle to the vertical effectively to prevent bubbling and frothing at the conuence of the water and metal streams. In an actual operating installation, a disposition of each nozzle 19 at an angle of 26 to the vertical was found to produce particularly satisfactory results as regards the formation of predominantly irregular and sharply angular powder particles. With the nozzles 19 disposed at such a small angle, the issuing water streams tend to join smoothly and descend as a single stream into the lower section of the housing.

The atomized particles of refined metal drop into the collecting chamber formed by the lower housing section 16 and are collected in the contained body of cooling water designated by the numeral 20. The powder particles are periodically (or continuously, if desired) removed frorn the collecting chamber by suitable means such as a pump 21 which pumps away the cooling water along with entrained powder particles.

For certain processes, and particularly for the production of high quality iron or steel bars and strip, it is desirable to effect vacuum degassing of the molten metal, and this is accomplished by placing the atomizing chamber under an appropriate vacuum, as by means of a vacuum pump 22. The application of a vacuum to the atomizing chamber 15 is advantageous in a number of respects. First, the exposure of the molten metal streams to the evacuated atomizing chamber causes the streams to literally burst apart, making it easier for occluded gases to be released from the metal. Second, the reduced ambient pressure within the chamber establishes a greater pressure differential relative to the vapor pressures of the gases to promote their release from the molten metal. Partial evacuation of the chamber 15 also significantly improves the efficiency of the atomizing operation by enabling increased areas of the metal to be initially contacted by the high pressure water jets and by causing the molten metal to be drawn through the opening 18 at a greater velocity and rate of flow than would be realized under corresponding conditions with gravity flow alone.

In addition to the evacuation of the atomizing chamber 15, or in place thereof, it may be desirable to introduce a controlled atmosphere into the vicinity of the atomized metal particles. In a typical installation, it may be desirable to introduce into the atomizing chamber an inert gas, such as argon, for example, so that the particles are enveloped in a controlled, inert ambient to prevent oxidation or nitrogen pick-up. Naturally, if the atomizing chamber is being maintained in an evacuated condition, the rate of ow of the controlled atmosphere in the chamber will be relatively low, so as not to entirely balance the effect of the evacuating pump 22. For this purpose, suitable regulating valve means 23 may be provided in the inlet pipe 24 for the controlled atmosphere.

In some instances, and particularly where vacuum degassing of the molten metal is practiced, it may be desirable and advantageous to introduce a reducing ambient atmosphere into the atomizing chamber. In such a case, hydrogen gas may be controllably introduced to combine with and neutralize the oxygen released during degassiication of the molten metal.

In an advantageous alternative form of atomizing chamber, shown in FIGS. 2 and 3, partial evacuation of the upper housing section 71 is effected through the action of the high pressure water streams 70 passing through an orifice 72 in a separator plate or diaphragm 73 which divides the top and bottom sections of the atomizer housing. The converging jets are karranged to meet in the region of the orifice 72, which is of a size and shape to closely accommodate the well-defined waterl streams.

As indicated in FIG. 1a, the cooling water and the iron powder particles entrained therein are discharged by the pump 21 through a conduit 25 and to a dewatering unit 26, which may be a conventional settling basin, filter, or centrifuge. Advantageously, the entrained particles are first passed through an apparatus, such as a separator 27, by means of which low density impurities, such as slag, furnace refractories, and the like, are removed from the higher density metal powder and discharged through an outlet 28.

The dewatered metal powder, which is still, of course, very wet (e.g., l percent to perhaps as high as l5 to 20 percent water content) and has a viscous consistency, somewhat like mud, is directed through a conduit 29 or otherwise to a drying and screening Ichamber 30, in which the powder is heated to a temperature in the region of 300 F. or over to effect water evaporation.

Advantageously, the iwater supplied to the system, for atomizing, cooling, and transporting of the iron powder, has suitable additives or treatments to reduce its gas content (principally oxygen and nitrogen). The water thus serves as a temporary protective ambient to prevent oxidation or nitriding of the powder during atomization and during the period it is submerged.

In accordance with one aspect of the invention, the iron powder introduced into the drying and screening chamber is exposed to a controlled, inert ambient, typically nitrogen or argon gas, for example, and is maintained in a controlled ambient Iuntil formation of a substantially nished sheet, strip, or bar, and its emergence from the process at a temperature below that at which oxidation readily occurs. Thus, referring again to FIG. la, an inert ambient atmosphere such as nitrogen is introduced into the drying and screening chamber 30 through a suitable conduit 31, so that the iron powder is exposed to the atmosphere during the drying process.

As the powder becomes dry, it is passed over suitable screening means (not specifically shown). In some cases, it may be desirable to classify the powder into various size ranges. However, in the process of the invention, it is usually more desirable to simply screen the particles to pass all those particles smaller than a given size and reject all those particles of greater size. Advantageously, all particles capable of passing through a 40 mesh screen are accepted as a group, and all larger particles are discharged for rework or discarding. It is desirable in the process of the invention to work with intermixed particles of various sizes, since the finer particles pack in between the larger particles and facilitate the compacting of the powder particles into a dense, coherent strip of metal.

In the continuous procedure of the invention, the dried metal powder particles passing through the screening chamber 30 are conveyed, advantageously by gas entrainment, through a conduit 32 to temporary holding bins 33, the latter being supplied with an inert atmosphere, such as nitrogen, as through an inlet conduit 34. The temporary holding bins 33 function to absorb temporary fluctuations in the rate of powder making and the rate of subsequent strip formation, as will be understood.

Associated with the outlet 35 of a holding bin is a blending chamber 36, in which the primary metal powder particles may be mixed and blended with desired additives, such as detergents, activators, lubricants, binders, or, in appropriate cases, other metal powders or alloying agents. The additives typically may be introduced through an inlet facility 37. Also, reducing atmospheres may be added which will become effective when the powder is later preheated for compacting.

The blending operation may be of particular significance in a typical process according to the invention, because of the advantageous controls provided over the final metal composition, as well as the ability to promote or facilitate certain of the subsequent operations. For example, the addition of appropriate detergents and activators can significantly reduce the times and temperatures required for subsequent heating and/or sintering operations. Further, the detergents and activators, as well as various desirable lubricants and binders, can greatly facilitate the operation of compacting the powder to form a green strip. Of perhaps even greater importance, however, the blending stage permits alloying powders to be mixed with the otherwise high purity metals to achieve a variety of advantageous effects, including the formation of alloys otherwise impossible or impractical to produce.

The metal powder can be compacted directly by being controllably fed to the compacting rollers 39, but the use of a preheating apparatus 38 will provide for increased production rates and for desirable operating exibility. Precise feed control is important in order to achieve a uniform rate of feed toward the compacting rollers 39 and to assure that the rate is uniform across the entire width of the compacting rollers. Where iron or steel making powder Ablends are employed in the procedure, the

feed control facility (not specifically illustrated) may include appropriate magnetic pump or roller means, for example.

In the preheating chamber 38, the powder particles are heated to a point at which the particles will tend to soften and plasticize, although the temperature should be maintained below that at which the powder mass will become too sticky to process. With a steel making blend of particles, an advantageous preheating temperature is in the region of 1000o F., or over. Experience indicates that the maximum temperature possible is advantageous, but this maximum temperature will vary with different metals or alloys and the methods used.

Advantageously, heat imparted to the powder particles during the drying stage is utilized to assist preheating, Where practicable. This is accomplished by delivering the newly dried powder promptly to the preheat stage lwhile maintaining the powder conveying and holding facilities insulated against rapid heat loss.

An advantageous form of preheating and feeding system is illustrated in FIGS. 4 and 5, which provides for a high rate of feed =while assuring uniform distribution as well as uniform heating of the powder. The equipment includes a. supply chamber 74 in which the powder is given a first stage preheat to as high as about 900 F. The partially preheated powder is then directed through a plurality of spaced distribution tubes 75 which, collectively, form an effectively continuous discharge outlet immediately above the nip of the compacting rollers 39.

The distributing tubes 75 are provided with heater units 77 which impart a second stage preheat to the powder, raising it (in the case of iron or steel making blend) to its final preheat temperature of 950 F. or higher. The distributing tubes, which may be on the order of l inch in diameter or less, provide for closely and individually controlled and uniformly effective heating of the powder.

As the powder increases in temperature, entrapped gases expand and must be removed. Accordingly, the system of the invention advantageously includes evacuating tubes 78 positioned concentrically iwithin the distributing tubes. They are arranged to efficiently remove air or gases displaced from the powder in the course of feeding, preheating, and compaction.

At the preheat temperatures used in the process, reducing gases previously added to the powder become effective in reducing oxides that may be present. Thus, the powder preheating operation, as carried out in accordance with the invention, serves not only to soften the powder for more advantageous compacting, but eliminates the need for annealing and oxide reduction operations normally performed in conventional powder production procedUreS.

Control of the liow of iron or steel powder through the distributing tubes may advantageously be effected through the use of means such as magnetic coil means (not shown) around the tubes. By establishing a downwardly travelling magnetic field, the powder will be, in effect, pumped downward. Magnetic means also may be used as Valves to effect individual control over the downward flow of powder. Any tendency of the hot powder to stick to the tubes can be reduced by the application of vibrators 79.

In the process of the invention, the preheated powder particles are compacted by the rollers 39 to a density in the range of 70 to 95 percent that of solid metal strip, and advantageously this is accomplished using compacting rollers having a diameter greatly in excess of the compacted strip thickness (for example on the order of l0() to 300v times the thickness of the initially compacted strip). The product emerging from the first stage of the compacting rollers 39 is referred to as a green strip. It is reasonably integrated and is self-supporting but is still quite weak relative to finished metal strip.

Following initial compaction, the green strip is diverted about a guide roller 41 and directed into an elongated heating chamber 42, in which the green strip is heated to a higher temperature, in the range of '1600 F. to 2200 F. The green strip, which may be partially sintered within the heating chamber 42 where desired, is in any event in a desirably heated condition for further compacting, upon its emergence, by means of final stage compacting rollers 43. The rollers 43 serve to compact the heated strip to substantially percent density.

The strip passing through the heating chamber 42, being in a porous condition and at high temperature, is ideally receptive to a variety of gas reaction treatments, such as carburizing, decarburzing, deoxidation, nitriding, chromanizing, nickelizing, etc. These reaction treatments may be advantageously carried out by introducing appropriate gases into the heating chamber or into selected, divided regions of the heating chamber. In this connection, the chamber may be made as long as is necessary and desirable to effect the necessary heating of the strip and its exposure to the reaction medium. Further, advantage may be taken of the heated, porous condition of the green strip within the heating chamber to cause the strip to be infiltrated with a lower melting point metal. Iron or steel strip, Ifor example, may be readily infiltrated with molten copper, such that the product emerging from the heating chamber is a substantially solid material of unique properties. Various additives from the blending stage also bring about advantageous effects. Detergents and activators promote sintering or hot compacting, and compounds such as dissociable hydrides release protective or treating gases in the irnmediate vicinity of the particles.

In the production of iron or steel strip in accordance with the invention, it is contemplated that the densified strip, indicated by the reference numeral 44 in FIG. lb, will be reduced to a substantially finished size or thickness, and one of more hot roll reduction stages 45, 46 advantageously are provided for this purpose, located immediately following the final stage compacting rollers 43, to receive the densified metal while it still retains the heat of the chamber 42. In this connection, the hotreduced form of a steel or iron strip or bar, designated by the numeral 47 in FIG. lb, may readily fall well within the size ranges conventionally achievable only by cold reduction processes. Thus, in the manufacture of iron and steel strip, following conventionally achievable only by cold reduction processes. Thus, in the manufacture of iron and steel strip, following conventional procedures, the practical lower limit of hot-rolled reductions is to a strip thickness of 0.060 inch, and even this lower range is very difficult to achieve. With the procedure of the invention, however, since the thickness of the fully densified strip 44 may be readily controlled at the rst compacting stage, the hot reduction may be carried out to minimum strip thickness on the order of 0.010 inch without difficulty. Similar advantages are realizable, of course, in the manufacture of substantially finished bars and rods.

In accordance with one aspect of the invention, the iron or other powder is maintained under an inert ambient from the time of its delivery as dried powder to the holding bin 33 to the time of its emergence as a substantially iinished product at a temperature below that at which oxidation will readily occur. To this end, it is appropriate to maintain the strip wholly enclosed in a suitable chamber 48 (or series of chambers) which, in effect at least, embraces the strip from the point of its initial formation to the point of its emergence at a relatively low temperature. The chamber 48 is supplied, as through a conduit 49, with a suitable inert atmosphere, such as nitrogen or argon. In this connection, it may be desirable to embrace the strip with a series of individual chambers, rather than a single large chamber as schematically illustrated in FIG. 1b, to achieve various practical conveniences and to minimize requirements of the gas forming the controlled ambient. Further, while nitrogen is a desirable gas for many stages of the strip forming process, it tends to react with iron or steel at higher temperatures, and other gases, such as argon or prepared atmospheres, may be desired for protecting or treating the strip during its passage through the heating chamber 42.

The strip 44 may be protected from oxidation as it travels from the furnace 42 to the cooling sprays 50 by flame curtains, which are reducing. However, it may be desirable in some cases to impart a controlled oxide coating on the strip surface.

Prior to the emergence of the substantially nished strip from its protective ambient, the strip may advantageously be subjected to cooling sprays 50, which serve to reduce the strip temperature to a range of about 300 F. to 400 F. At this temperature, there is very little tendency for iron or steel strip to oxidize.

The cool, substantially finished product is typically directed to a rolling stage 51, for cold reduction, for temper rolling, or for desired surface characteristics. Thereatfer, the iinished product is directed toa flying shear S2, for example, for cutting into sheets, finite bars, etc., or to a coiler, indicated at 53, for coiling into longer, continuous lengths. Typically, two coilers would be employed to accommodate uninterrupted operation.

In the event that the production of powder exceeds the output of the product-forming end of the continuous process, or where otherwise desirable and expeditious, some of the powder discharged from the drying and screening chamber 30 may be diverted out of the system and bagged or otherwise stored for subsequent use. To this end, the system may include an auxiliary conduit 54, control valve means 55, and a bagging or other storage installation, schematically indicated at 56. Advantageously, the stored or bagged powder is maintained under a controlled ambient for prevention of oxidation. This is usually done by inserting moisture absorbing material such as packages of silica gel.

In some instances it may be desirable or advantageous to produce quantities of powder as an end product of a particular installation, for resale to customers or otherwise for subsequent use in various ways. In such cases the dewatered or dried powder is subjected to a predetermined annealing step prior to packaging, so that the powdered end product has desirable softness to accommodate subsequent compaction of the powder into metal forms in conventional ways. Typically, the annealing step can be carried out in a controlled atmosphere, but it is possible and practicable, and in many cases desirable and advantageous, to combine annealing with chemical treatment. By way of example, where particularly low carbon 'content is sought, the anneal may be carried out in a wet atmosphere to bring about carbon-reduction reactions. The metal, being in powder form, presents to the reactive gas a large surface area which facilitates the desired chemical interaction. For some applications it may even be desirable to form a controlled oxide coating on the iron powder, in which case the anneal may be carried out all or in part in an oxidizing atmosphere.

In the manufacture of iron powder, the annealing step may be carried out by heating the powder to a temperature on the general order of 1100 F. to 1350 F.

In the procedure of the invention, annealing is of particular significance because the atomization procedures inherently tend to result in hardened powder particles, which without treatment are less suitable for compaction than desired, particularly with respect to batch compacting operations, utilizing platen press equipment. The powder of the new process, in addition to having a desired, controlled analysis and an advantageous particle shape, by reason of refining and controlled atomization of the molten metal, has, after annealing, the softness more characteristic of costly electrolytic and carbonyl iron powders. ln addition, powder resulting from the new process may have special desired char acteristics imparted by means of a reactive annealing operation.

The process of the invention, while having applicability to a number of metals, is especially advantageous for the conversion of molten iron or steel to iron or steel products, such as strip, because of the significant economic and procedural advantages which, in the case of the extremely high temperatures involved in the manufacture of iron and steel, are of critical practical signicance usefulness in connection with the production of copper or nickel products and of products formed of alloys of 50 percent or more copper, nickel, or iron.

In a typical operation according to the invention in which the starting raw material' is scrap metal, the analysis of non-ferrous elements in the starting material may be somewhat as follows:

Percent Carbon 0.18 Manganese 0.57 Sulfur 0.032 Silicon 0.04 Chromium 0.01 Molybdenum 0.01 Copper 0.03 Phosphorous 0.015

A starting material of the above analysis would advantageously be rened in the vessel 11 to a condition of high purity such that the powder product formed with the refined steel has an approximate analysis, after annealing, as follows:

The resulting low carbon powder is useful and indeed especially desirable in its subsequently produced strip form for small electrical motor manufacture, because of the particularly good magnetic properties of the strip, coupled with its low cost. Even greater advantages are realized for this purpose when a controlled surface oxide coating is imparted to the strip.

By using selected quality raw materials, such as pig iron of preferred analysis, powder of even greater purity can be produced without difficulty. A typical approximate analysis of such a higher purity powder is as follows:

Percent Carbon 0.015 Manganese 0.008 Sulfur 0.020 Silicon 0.038 Chromium 0.000 Molybdenum 0.001 Copper 0.025 Phosphorous 0.010 Oxygen 0.320 Nitrogen 0.010 Acid insolubles 0.025 Iron 99.30 Sieve analysis (through 80 mesh sieve) 98.00

As one of the specific but significant aspects of the invention, iron strip made in accordance with the abovedescribed procedures may be adapted particularly for electrical applications, in the manufacture of laminated cores for small motors, transformers, solenoids, and the like. The manufacture of strip for electrical purposes is a substantial segment of the iron and steel industry, which involves important tonnage requirements of strip. This strip is specified for desirable magnetic properties, in addition to appropriate forming characteristics. The desired forming characteristics typically involve good punchability and may also involve uniform gauge, fiatness, desired grain and crystallographic structure, softness, resistance to cold welding, etc.

Iron electrical strip manufactured in accordance with the process of the invention is particularly adaptable to electrical end uses (although not, of course, limited thereto) because, among other reasons, of the ability under the new process to control both the nature and extent of impurities and to control the size of grain. Probably the most important factors affecting the magnetic properties of iron powder and strip (as well as the ability of iron powder to compact into strip) are the carbon and nitrogen contents of the metal. In accordance with the procedures of the invention, both nitrogen and carbon can be controlled with precision and reduced to very low levels without incurring extraordinary manufacturing expenses. Moreover, such of the impurities as are present are largely in the form of fine boundary precipitates which do not adversely affect the magnetic properties as do dissolved impurities; rather they tend to impart desirable properties to the strip, as by improving punchability and lowering eddy current losses. The advantages of carbon and nitrogen control are, of course, realized in all end uses of the powder, because ability to compact the powder into strip or other solid forms to maximum density with minimum pressure is one of the most important characteristics of the powder, but the advantages are of particular significance in connection with electrical strip because of the desirable magnetic and other characteristics realized through the control of both the amount and the nature of carbon and nitrogen in the manufacture of powder and the strip.

In the procedure of the invention, nitrogen content can be reduced and controlled by a variety of procedural steps, many or all of which advantageously would be varied in a particular operation. Thus, in the refining stage, nitrogen content is removed by flushing and by the addition to the molten bath of iron ore or iron oxides, to bring about a so-called carbon boil which accompanies reduction of the ores and oxides. Regardless of the utilization of the carbon boil procedure, the molten metal, during both its containment in the refining vessel and its atomization can `be protected from the nitrogen content of the atmosphere, as by formation of a slag surface layer on the bath in the refining vessel and/or the pro` vision of a suitable inert (e.g., argon gas) atmosphere during refining, pouring, and atomizing. The atomizing medium itself, which is advantageously water, can be desirably treated to remove as much as possible of the dissolved or contained nitrogen or nitrogen compounds, so that the metal remains protected even after immersion in the quenching liquid. If, after the atomizing step, the powder still analyzes to a higher nitrogen content than desired, a further reduction of nitrogen content may be carried out during an annealing step or during sintering of the powder, compact, or porous strip, as by in a hydrogen atmosphere.

Carbon content can be readilly controlled in accordance with the new process, first by appropriate selection of metal for the melting and refining procedure and, in conjunction therewith, adding carbon to or removing it from the bath of molten metal during refining, in accordance with well-known procedures. Where appropriate, further reduction in the carbon content can be brought about in the atomizing stage, as by atomizing the powder in an evacuated atmosphere to reduce the carbon monoxide content of the metal. Thereafter, additional carbon can be removed and controllably from the powder, compact or strip during subsequent annealing or sintering operations.

In a typical process according to the invention, one may without difiiculty produce electrical strip having a carbon content of less than 0.02 percent and a nitrogen content of less than 0.01 percent. These levels are eminently suitable for typical electrical strip applications. In general, it would appear that the combined carbon and nitrogen content of the iron should not significantly exceed a total of 0.04 percent in an electrical strip made in accordance with the invention, and experience demonstrates that these irnpurities are easily maintained well below this indicated upper limit.

ln instances where electrical strip of improved electrical characteristics is desired, collateral difficulties may arise with respect to the punchability of the strip, because of the relative softness of the finished, high purity strip. In such a case, it may be desirable and advantageous to impart to the surface of the powder particles (but not homogeneously to the entire body of the metal) trace coatings of such impurities as oxides, nitrides, phosphorous or manganese. These surface impurities significantly improve the shear properties of the strip and impart good punching characteristics, but are not significantly deterimental to the electrical properties of the finished strip.

Another feature of this process that improves the punchability of laminations is that the strip can be produced at less than percent density. Controlled compacting and processing of strip can produce soft annealed strip containing many small pores or voids. The presence of these improve the shearing action of the punch as it passes through the strip.

Trace coatings of impurities on the powder particles and small voids in the strip either individually or together make possible the production of strip which can be given a final heat treatment and thereafter punched and used. The normal practice is to punch the lamination from a strip, heat treat 'by a less effective method, and then use. Conventional strip that has received its final heat treatment cannot be punched satisfactorily.

It also may be desirable to impart a controlled oxide coating to the surface of the finished electrical strip, to provide a desired measure of electrical insulation between adjacent layers of a laminated stack.

Quite apart from the ease of controlling the content of significant impurities, such as carbon and nitrogen, the procedure of the invention is advantageous in the manufacture of electrical strip because of the substantially simplified procedures for the formation of the strip. Under conventional procedures, for example, the metal is refined in the usual manner and poured into ingots, which are subsequently rolled into slabs. Typically, the slabs are then hot rolled and coiled in continuous hot strip mills and later pickled to remove surface oxides. Thereafter, the hot rolled strip may undergo a plurality of cold reduction steps. each followed by annealing, after which the cold reduced strip is given a final high temperature anneal.

In contrast, the procedure of the invention enables the metal to be formed initially at controlled, minimum strip thickness, from which it may be directly hot reduced to a usable thickness. Not only does this avoid significant manufacturing steps, involving substantial time and plant installation, but it also affords desirable control of internal stress and grain formation which have a significant effect upon the electrical properties of the nished product.

In a typical specific procedure for the preparation of electrical strip for routine applications, a desirable raw material would be in the form of a light scrap iron, most advantageously taken from punching operations and electrical lamination plants, b-ut suitably also from other supplies of plain carbon steel scrap. This scrap may be melted in an electric furnace, under conditions substantially excluding contact of the metal with nitrogen, as by melting under an atmosphere of argon or under a slag blanket. Typically, melting and 'refining of the metal is carried on to the point where the carbon content of the melt is relatively low, advantageously 0.13 percent or less. The rened molten metal is atomized in the manner hereinabove described, with particular care being taken to minimize exposure of the molten metal to nitrogen-containing atmosphere or nitrogen-containing cooling water. Although some slight nitrogen content, as in the form of surface nitrides, may be advantageous, deleterious amounts of dissolved nitrogen will be present unless precautions are taken at this stage to avoid excessive exposure of the molten metal to nitrogen (and also at subsequent stages to avoid exposure of the powder to nitrogen when the powder is at a temperature of around 1350 F. or above).

After atomization, the relatively nitrogen-free powder is dewatered. If desired, the drying operation can be, in effect, an annealing and an extended chemical heat treatment, to reduce the carbon and oxygen contents to relatively minimum levels of about 0.005 percent carbon and 0.05 percent oxygen. At the other extreme, the dewatering operation may simply lower the water content to about percent moisture, with the chemical heat treatment being carried out in subsequent operations.

The dried powder is screened to remove oversized particles, typically leaving a balance of particles predominantly in the size range of about minus 40 mesh. The screened particles may then be treated by milling, tumbling, blending, etc., if desired, to achieve certain characteristics of iiowability, compressibility, and apparent density, and the powder at this stage may also be comfbined with so-called detergents. If particularly high quality electrical strip is being produced, it may be done in two ways: Elements such as silicon or a silicon compound, normally ferro silicon, can be included in the molten metal so that the powder produced is an iron alloy of desired analysis. A typical example of silicon-iron alloy powder that has been produced is as follows:

Percent Carbon 0.13 Manganese 0.33 Sufur 0.03 Silicon 1.3 5 Phosphorous 0.01 Nitrogen 0.029

In the subsequently produced strip, the carbon-was re` duced to 0.015 percent and the nitrogen to 0.015 percent. The second way is to add to the iron powder agents calculated to increase the electrical properties of the mix, suitable such agents being electrolytic silicon metal powder, ferro silicon, or certain organic silicon compounds, such as silane compounds. It should be particularly noted that additions if utilized may be introduced into the atomized and solidified powder as well as into the original melt.

The dried electrical powder, preheated if appropriate, is charged into the rolling stand for compacting and hot rolling generally in the manner previously described. It desired, the compacted and/or partially hot rolled strip may be given a chemical heat treatment, advantageously at a stage at which its density is on the order of to 98 percent of theoretical iron density. The heat treatment is continued as long as necessary to bring the carbon, oxygen, and nitrogen contents down to desired levels. Strip made from iron powder (without silicon or other additions) can acquire a magnetic induction of 15 kilogausses at magnetizing forces as low as l0 RMS ampere turns per inch, with losses of no more than 5.5 watts per pound. This is as compared to conventional non-silicon electrical steel, which requires magnetizing forces as high as 15 RMS ampere turns per inch, with losses as high as 7 watts per pound to reach inductions of l5 kilogausses.

In the chemical heat treatment of the compacted and/ or rolled strip, the treating atmosphere advantageously contains considerable water vapor to reduce the carbon content of the strip and is reducing in order to react with oxygen present. Powder or porous strip is so reactive to gases that powder with carbon content of 0.38 percent has been reduced to 0.07 percent in a normal powder anneal and this same powder (0.38 percent carbon) when compacted into a strip and sintered (without annealing of the powder) had a carbon content as low as 0.02 percent.

In one typical procedure according to the invention, the prepared powder is compacted, then subjected to heating, advantageously in conjunction with a chemical heat treatment. Thereafter, and as part of the continuous procedure, the compacted and treated powder is hot rolled to a thickness equal to or closely approaching the iinal gauge. In other instances it may be desirable to perform chemical heat treatment in another, separate stage, in which event the prepared powder may be compacted, heated and then hot rolled into coils. Subsequently, the hot rolled coils are given appropriate chemical heat treatments or other treatments and they may then be cold rolled to a final gauge. If cold rolling is involved in the procedure, a subsequent annealing step normally would be performed.

The electrical strip manufactured in accordance with the above-described procedure is especially desirable because significantly improved electrical and mechanical properties are achieved without increase in cost. The particularly desirable properties of the material are realized in the process of the invention because the electrical strip is formed with relatively large grains which are extremely pure internally to impart desirable softness and permeability and desirably high saturation magnetization and low coercive force, but have some boundary impurities in the form of nitrides, oxides, and other precipitates, initially formed on the surfaces of the powder particles and eventually forming and retentively defining the grain boundaries. The result of this structure is to provide large individual grains of pure iron separated by high resistivity boundaries to limit eddy current losses. The impurity-defined grain boundaries also impart highly desirable shear properties between the individual grains, to provide signicantly improved punchability of the strip. This property can also be improved by producing strip with pores which reduces the strip to a density of less than 100 percent. The property of good punchability is of substantial practical importance, since the magnetic structures formed of electrical strip typically are comprised of laminated stacks of punched-out shapes.

The described grain boundary characteristics of the particles and porosity of the strip also are of significance in imparting increased resistivity between the laminated elements and in reducing any tendency of the otherwise relatively soft iron strip sections to cold weld to the punching dies or to adjacent strip sections.

Although electrical strip produced in accordance with the invention may have quantities (e.g., up to about 5 to 6 percent) of silicon added, where particularly high quality electrical properties are sought, it is possible t0 o'btain, in accordance with the invention, electrical properties in substantially silicon-free carbon steel strip which are achievable in conventional materials only through the addition of silicon or other similar additives. In addition, the new strip has significant collateral advantages of a mechanical nature such as ease of stacking and improved punchability.

The combined properties of the completed strip for electrical applications are uniquely advantageous in that the combination of large, highly pure particles surrounded and separated -by boundary coatings in the form of relatively trace quantities of nitrides, oxides, and other compounds, as well as containing a desired amount of porosity, simultaneously imparts to the finished strip electrical and mechanical properties which heretofore have been considered relatively mutually exclusive. The large, pure particles result in desirable electrical characteristics of high sautration magnetization, high permeability, low coercive force and accompanying low hysteresis loss. This strip possesses the unique quality of satisfactory punchability after the final heat treating operation for maximum electrical qualities. This is not feasible with strip made in the conventional manner.

Boundary precipitates, in the relatively minor quantities present, and/or the controlled porosity impart very desirable characteristics, both mechanical and electrical, to the strip. Electrically, the precipitates, in the form of nitrides, oxides, and other electrically resistant compounds, serve to limit the flow of current between particles, so that the eddy currents tend to be confined within the individual particles or grains, a condition which significantly reduces eddy current losses.

Mechanically, the boundary precipitates and pores are advantageous in improving the shear properties of the metal, relative to conventional metals of similarly low impurity content, so that the desired punchability characteristics are realized without subjecting the metal to :work hardening or other treatments, which not only add cost but introduce other undesirable side effects such as internal stresses. Good punchability requires low ductility `without Ibrittleness, a well dened yield point, and a minimum tendency to weld cold, and all of these conditions are relatively optimized in the material of the present invention by reason of the unusual grain structure of the compacted strip, consisting of large, pure particles with grain boundaries defined by fine trace precipitates of relatively brittle carbon, nitrogen, and oxygen cornpounds and/or pores in the strip.

In the manufacture of certain other steel products for general applications, as well as for electrical strip, it is especially significant in the process of the invention that the intermediate stage of the metal is in the form of low carbon iron or steel powder particles, rather than in the conventional form of ingots at one intermediate stage and slabs, blooms, or billets at another intermediate stage. With the procedure of the invention, the same intermediate material-powder of controlled, desired analysismay be utilized in the formation of strip, bars, rods, etc., and intermediate handling and storage operations are reduced to an ultimate minimum, as compared to conventional steel making operations. The process of the invention also enables the production in commercial quantities of many steel products which heretofore have been unable to be produced, at least otherwise than on a laboratory basis. One important class of steel products which may be produced according to the new process is hotrolled strip in gauges of less than 0.060 and well down into the range conventionally available as the more expensive cold-rolled strips.

One of the important specific features of the invention resides in the provision of an atomizing facility, particularly for iron but also useful for other metals, in which solid hat sheets of quenching liquid are directed at high pressure into a V-shaped intersecting relationship. The molten metal, in one or more small diameter streams, is directed into the trough of the V, where it is violently impacted by the solid water streams and is thereby quenched and atomized in the form of sharply angular particles. In this respect, iron in particular has a high surface tension and thus a very strong tendency to form spherical atomized particles, which are quite useless as a practical matter. The use of high pressure solid flat sheets of quenching liquid, intersecting in a narrow V, as distinguished from other forms of quench liquid application, appears to be significantly advantageous and in some cases may be critical to practical operations. Pressures of upwards of 400-500 p.s.i. are usually most advantageous, although under some circumstances somewhat lower pressures can be used. In any event, the pressure should be sufficiently high to assure formation of sharp, angular particles and to avoid rounded or spherical particles.

The ability to produce sharp, irregular iron particles enables the particles to be compacted effectively into a coherent strip, to achieve highly advantageous end products such as the above-described electrical strip. In this respect, the finally consolidated strip is characterized by an especially desirable grain structure which is not obtainable by more conventional procedures.

The new feeding arrangements, for controllably feeding metal powder into the compacting rollers, are advantageous in that they can assure controlled uniformity of powder feed across the width of the strip, even at high lineal speeds of strip consolidation. The feeder also incorporates advantageous arrangements for preheating the powder immediately upstream of the compacting rollers. If desired the preheating stage may include the release of reducing gases, for a final reduction of oxides which may be present.

It should be understood that the specific forms of the invention herein illustrated and described are intended to be reprsentative only and that certain changes may be made therein without departing from the clear teachings of the disclosure. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention. In the claims and elsewhere herein references to electrical strip is not intended as a limitation on the use thereof, but only as a characterization that the strip may be useful for electrical applications. Strips having such'characteristics will, of course, have many other utilizations.

I claim:

1. A metallic strip, consisting essentially of (a) a body of iron particles consolidated together under heat and pressure and forming a self-supporting striplike section,

(b) said particles having a sharply angular irregular configuration,

(c) said strip having a combined carbon and nitrogen content less than about 0.04 percent,

(d) said strip having an induction of about 15 kilogausses with losses of not more than about 5.5 watts per pound when subjected to a magnetizing force of not more than about 10 RMS ampere-turns per inch.

2. The metallic strip of claim 1 further including (a) up to about 6% by weight of silicon.

3. The metallic strip of claim 1, wherein (a) at least some of said iron particles are coated with surface impurities having electrical resistive properties,

(b) whereby said coating of surface impurities forms 4. The metallic strip of claim 3, wherein said surface impurities having electrical resistive properties are chosen from the group consisting of oxides, nitrides, phosphorous and manganese.

References Cited UNITED STATES PATENTS Kelley 75-222 X Whitehouse 75-224 X Whitehouse 75-.5

Von Dhren 75-222 X Storchheirn 29-182 X Storchheirn 75--224 X Freeman 29-182 X Adams 75-200 X Kreiselmaier 75--200 X Lindstrm 75-224 X Schlecht 75-.5

Beller 75-.5

Hansen 75--.5

Yamazaki 75-.5

18 3,194,858 7/1965 Storchheirn 75-226 X 3,348,982 10/1967 Dunton 75-.5 X

FOREIGN PATENTS 398,045 9/1933 Great Britain. 782,688 9/ 1957 Great Britain.

OTHER REFERENCES CARL D. QUARFORTH, Primary Examiner ARTHUR I. STEINER, Assistant Examiner U.S. Cl. X.R.

gyggg UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No 3 ,502,1411-6 Dated March 2Q, 1970 Inventor(s) Maurice D- Ayers It ie certified that error eppeere in the above-identified potent and that seid Letters Patent ere hereby corrected es chown below:

' Column 2, line 140, "eddtives" .should-read edditivee.

Column E2,l line im, "stickly" should read "sticky". Column lt, line 2, re:l.n:roduces'I should read reintroduce. `Column 7, line l "distribution" should real "distributing". Column 8, line 5 "of" should' reed "or". Column 8, lines 67-69, delete "Thus, in the manufacture o1 iron and steel strip, following conventionally achievable only by cold reduction processes. Column 9, line Mt, "Thereatfer" should reed Thereerter. Column l0, line 17, after "operations" delete the egmmaf line giy .-sxnifcah-eu should read nit1cenoe. It is s61 of significant". Column 10, line 38, "metal" should reed steel. Column ll, line 48, "the im rities" should reed --these impurities. Column ll, line 7 after "etomizetion" insert a comme.. Column l2, line l2, before "compact" insert "powder". Column 12, line l2, after "by" insert treatment. Column l2 line 214,' otter "removed" insert easily. Column 13, line "steps. should read steps,. Column 13, line 7, before "controlled" insert -a-. Column l5, line 24, "seutration" should read seturation.

l'T- hn- .vgfl No: s 'im (SEAL) '"f' Attest:

Edward lf. i-f m* le m nesting 0mm Mmmm f um"

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