CASTING IN AN EXOTHERMIC REDUCING FLAME ATMOSPHERE

Abstract: An apparatus and method for casting metal strip include a moveable chill body (7) having a quench surface (5) thereon. A nozzle mechanism (4) deposits a stream of molten metal onto a quenching region (14) of the quench surface to form the strip (6), and a gas supply mechanism (8, 12) provides an initial gas mixture, which consists essentially of carbon monoxide and oxygen. An ignition mechanism ignites the initial gas mixture to create an exothermic reaction which provides a low density reducing flame atmosphere at a depletion region (13) located substantially adjacent to and upstream from the quenching region. A control mechanism (16) controls the initial gas mixture to produce an adjusted reducing flame atmosphere at the depletion region in which the adjusted reducing flame has a burnt gas composition that includes substantially no free oxygen.

Description: DESCRIPTION CASTING IN AN EXOTHERMIC REDUCING FLAME ATMOSPHERE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the casting of metal strip directly frαn a melt, and more particularly to the rapid solidification of metal in a flame atmosphere directly from the melt to form substantially continuous metal strip. 2. Description of the Prior Art U.S. Patent No. 4,142,571 issued to M. Narasimhan discloses a conventional apparatus and method for rapidly quenching a stream of molten metal to form con- tinuous metal strip. The metal can be cast in an inert atmosphere or a partial vacuum. U.S. Patent No. 3,862,658 issued to J. Bedell and U.S. Patent No. 4,202.,404 issued to C. Carlson disclose flexible belts empToyed to prolong contact of cast metal filament with a quench surface. The casting of very smooth strip has been difficult with conventional devices because gas pockets entrapped between the quench surface and the molten metal during quenching form gas pocket defects. These defects, along with other factors, cause considerable roughness on the quench surface side as well as the opposite, free sur¬ face side of the cast strip. In some cases, the surface defects actually extend through the strip, forming per¬ forations therein. U.S. Patent No. 4,154,283 to R. Ray et al. dis¬ closes that vacuum casting of metal strip reduces the formation of gas pocket defects. The vacuum casting system taught by Ray et al. requires specialized cham¬ bers and pumps to produce a low pressure casting atmo- sphere. In addition, auxiliary means are required to continuously transport the cast strip out of the vacuum chamber. Further, in such a vacuum casting system, the strip tends to weld excessively to the quench surface instead of breaking away as typically happens when cast¬ ing in an ambient atmosphere. U.S. Patent No. 4,301,855 issued to H. Suzuki et al. discloses an apparatus for casting metal ribbon wherein the molten metal is poured from a heated nozzle onto the outer peripheral surface of a rotary roll. A cover encloses the roll surface upstream of the nozzle to provide a chamber, the atmosphere of which is evacu¬ ated by a vacuum pump. A heater in the cover heats the roll surface upstream from the nozzle to remove dew droplets and gases frcm the roll surface. The vacuum chamber lowers the density of the moving gas layer next to the casting roll surface, thereby decreasing forma¬ tion of air pocket depressions in the cast ribbon. The heater helps drive off moisture and adhered gases from the roll surface to further decrease formation of air pocket depressions. The apparatus disclosed by Suzuki et al. does not pour metal on.to the casting surface until that surface has exited the vacuum chamber. By this procedure, com¬ plications involved in removing a rapidly advancing rib¬ bon from the vacuum chamber are avoided. The ribbon is actually cast in the open atmosphere, offsetting any potential improvement in ribbon quality. U.S. Patent No. 3,861,450 to Mobley, et al. dis¬ closes a method and apparatus for making metal fila¬ ment. A disk-like, heat-extracting member rotates to dip an edge surface thereof into a molten pool, and a non-oxidizing gas is introduced at a critical process region where the moving surface enters the melt. This non-oxidizing gas can be a reducing gas, the combustion of which yields reducing or non-oxidizing combustion products at the critical process region. In a particu¬ lar embodiment, a cover composed of carbon or graphite encloses a portion of the disk and reacts with the oxy¬ gen adjacent the coyer to produce non-oxidizing carbon monoxide and carbon dioxide gases which can then sur¬ round the disk portion and the entry region of the melt. The introduction of non-oxidizing gas, as taught by Mobley, et al. , disrupts and replaces an adherent layer of oxidizing gas with the non-oxidizing gas. The con¬ trolled introduction of non-oxidizing gas also provides a barrier to prevent particulate solid materials on the melt surface from collecting at the critical process region where the rotating disk would drag the impurities into the melt to the point of initial filament solidifi¬ cation. Finally, the exclusion of oxidizing gas and floating contaminants from the critical region increases the stability of the filament release point from the rotating disk by decreasing the adhesion therebetween and promoting spontaneous release. Mobley, et al., however, address only the problem of oxidation at the disk surface and in the melt. The flowing stream of non-oxidizing gas taught by Mobley, et al. is still drawn into the molten pool by the viscous drag of the rotating wheel and can separate the melt frcm the disk edge to momentarily disturb filament for- . mation. The particular advantage provided by Mobley, et al. , is that the non-oxidizing gas decreases the oxida¬ tion at the actual point of filament formation within the melt pool. Thus, Mobley, et al. fail to minimize the entrainment of gas that could separate and insulate the disk surface from the melt. U.S. Patent No. 4,282,921 and U.S. Patent No. 4,262,734 issued to H. Liebermann disclose an apparatus and method in which coaxial gas jets are employed to reduce edge defects in rapidly quenched amorphous strips. U.S. Patent No. 4,177,856 and U.S. Patent No. 4,144,926 issued to H. Liebermann disclose a method and apparatus in which a Reynolds number parameter is con¬ trolled to reduce edge defects in rapidly quenched amor¬ phous strip. Gas densities and thus Reynolds numbers, are regulated by the use of vacuum and by employing lower molecular weight gases. Conventional methods, however, have been unable to adequately reduce surface defects in cast metal strip caused by the entrapment of gas pockets. Vacuum casting procedures have afforded some success, but when using vacuum casting, excessive welding of the cast strip to the quench surface and the difficultly of removing the cast strip from the vacuum chamber have resulted in lower yields and increased production costs. As a result, conventional methods have been unable to provide a commercially acceptable process that efficiently pro¬ duces smooth strip with consistent quality and uniform cross-section. SUMMARY OF THE INVENTION The invention provides an apparatus and method for efficiently casting smooth metal strip and substantially preventing the formation of gas pocket defects therein. Generally stated, the apparatus of the inven¬ tion includes a moveable chill body having a quench sur¬ face thereon. A nozzle means deposits a stream of molten metal onto a quenching region of the quench sur- face to form the strip, and a gas supply means provides * an initial gas mixture, which consists essentially of carbon monoxide and oxygen. An ignition means ignites the initial gas mixture to create an exothermic reaction which provides a low density, reducing flame atmosphere at a depletion region located substantially adjacent to and upstream from the quenching region. A control means controls the initial gas mixture to produce an adjusted reducing flame atmosphere at the depletion region in which the reducing flame has a burnt gas composition that includes substantially no free oxygen. In accordance with the invention there is also pro¬ vided a method for casting continuous metal strip. A chill body having a quench surface is moved at a selected speed, and a stream of molten metal is depos¬ ited on a quenching region of the quench surface to form the strip. An initial gas mixture consisting essen¬ tially of a carbon monoxide and oxygen is supplied and ignited to create an exothermic reaction which provides a low density, reducing flame atmosphere at a depletion _ 5 _ region located substantially adjacent to and upstream from the quenching region. The initial gas mixture is controlled to produce an adjusted reducing flame atmo¬ sphere, which contains substantially zero free oxygen, at the depletion region. The method and apparatus of the invention advanta¬ geously minimize the formation and entrapment of gas pockets against the quenched surface during the casting of the strip. As a result, the invention avoids the needs for complex vacuum casting apparatus and can be practiced in an ambient atmosphere. The exothermic reaction of the reducing gas in the depletion region surprisingly provides better and more uniform cooling and quenching of the molten metal. Heat resulting from the exothermically reacting gas provides a low density reducing atmosphere that inhibits the formation of gas pockets which operate to decrease contact between the molten metal and the quench surface. The more uniform quenching, in turn, provides improved physical proper- ties in the cast strip. In particular, the reduction of surface defects on the quenched surface side of the strip increases the packing factor of the material and decreases localized stress concentrations that can cause premature mechanical failure. The smoothness of the free surface side of the cast strip (i.e. the side not in contact with the quench surface of the chill body) is also improved by the method and apparatus of the inven¬ tion. This increased smoothness further increases the packing factor of the material. In the production of amorphous metal strip, the more uniform quenching afforded by the low density reducing atmosphere provides a more consistent and uniform formation of the amorphous state. In the manufacture of strip composed of magnetic material, the number and size of strip surface discon- tinuities is reduced, improving the magnetic properties of the strip. Surface defects due to entrapped gas pockets are reduced, and there is much less chance for a gas pocket — _fi- 86/00758 tO perforate the strip. Surprisingly, very thin strips (less than about 15 micrometers in thickness) have been produced. These very thin strips are highly desirable in various applications. For example, in magnetic devices, such as inductors, reactors and high frequency electromagnetic devices, thin magnetic material substan¬ tially reduces power losses therein. In brazing, the use of thinner brazing foils substantially improves the strength of the brazed joints. Moreover, the reduction of entrapped gas pockets markedly increases the heat conductive contact between the molten metal and the quench surface. Thicker strips of rapidly solidified metal can be produced. Such thicker strip is desirable because it can be more easily substituted for materials conventionally used in exist¬ ing commercial applications. These thick strip compon¬ ents can, surprisingly, be provided by rapid solidifi¬ cation in a single quenching step in much less time with decreased cost. Thus, the present invention effectively minimizes gas pocket defects on the strip surface which contacts the quench surface, and produces strip having a smooth surface finish and uniform physical properties. Complex equipment and procedures associated with vacuum casting are eliminated. The invention efficiently casts ultra thin as well as extra thick metal strip directly frcm the melt at lower cost and with higher yield. Such ultra thin and extra thick strips are especially suited for use in such applications as magnetic devices and can be substituted for conventional materials with greater effectiveness and economy. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood and further advantages will become apparent when reference is- made to the following detailed description of the preferred embodiment of the invention and the accom¬ panying drawings in which: _-_ FIG. 1 shows a representative prior art apparatus for rapidly casting metal strip; FIG. 2 shows a schematic representation of a embo¬ diment of the invention which employs an endless casting belt; FIG. 3 shows an embodiment of the invention which employs a gas delivery means located coaxial with a casting nozzle; FIG. 4 shows an embodiment of the invention which employs a rotatable casting wheel; FIG. 5 shows an embodiment of the invention which employs a flexible hugger belt to prolong contact of the cast strip with the quench surface; FIG. 6 shows a gas velocity profile at the quench surface portion on which molten metal is deposited; FIG. 7 shows a photograph of the quench surface side of strip cast in air on a beryllium copper sub¬ strate; FIG., 8 shows a photograph of the quench surface side of a strip cast in a carbon monoxide reducing flame on a beryllium copper substrate; FIG. 9 is a graph which representatively shows burnt gas composition and maximum flame temperature (calculated and measured) as a function of the vol. % of CO in an initial gas mixture composed of CO and oxygen; FIG. 10 is a graph which representatively shows burnt gas composition and maximum flame temperature (calculated and measured) as a function of the vol. % of CO in an initial gas mixture composed of CO and ambient air; FIG. 11 is a photomicrograph which representatively shows the quench surface side of a strip cast in a CO flame that contains excessive free oxygen; and FIG. 12 is a photomicrograph which representatively shows the quench surface side of a strip cast. in a CO flame that contains substantially zero free oxygen. — σ — DESCRIPTION OF PREFERRED EMBODIMENTS For the purposes of the present invention and as used in the specification and claims, a strip is a slen¬ der body the transverse dimensions of which are much smaller than its length. Thus, a strip includes wire, ribbon, sheet and the like of regular or irregular cross-section. The invention is suitable for casting metal strip composed of crystalline or amorphous metal and is parti- cularly suited for producing metal strip which is rapidly solidified and quenched at a rate of at least about 10 °C/sec from a melt of molten metal. Such rapidly solidified strip has improved physical proper¬ ties, such as improved tensile strength, ductility and magnetic properties. FIG. 1 shows a representative prior art device for rapidly casting continuous metal strip. Molten metal alloy contained in crucible 2 is heated by a heating element 3. Pressurization < f the crucible with an inert gas forces a molten stream through a nozzle 4 at the base of the crucible and deposits the molten metal onto a moveable chill body, such as rotatable casting wheel 1. Solidified moving strip 6, after its break-away point from the quench wheel is then routed onto a suit- able winding means. Ouench surface 5 (substrate) is preferably a mate¬ rial having high thermal conductivity. Suitable mate¬ rials include carbon steel, stainless steel and copper based alloys such as beryllium-copper. To achieve the quench rates of at least about 10 °C per second, wheel 1 is internally cooled and rotated to provide a quench surface that advances at a speed ranging from about 100- 4000 meters per minute. Preferably, the quench surface speed ranges from about 200 - 3000 meters per minute. Typically, the thickness of the cast strip ranges from 25-100 microns (micrometers). FIG. 2 shows a representative apparatus of the invention. A moveable chill body, such as endless cast- -9- ing belt 7, has a chilled casting quench surface 5. Nozzle means, such as nozzle 4, deposits a stream of molten metal onto a quenching region 14 of quench sur¬ face 5 to form strip 6. Nozzle 4 has an orifice 22 located at exit portion 26. A depletion means, includ¬ ing gas nozzle delivery means 8, and gas supply 12, sup¬ plies a reducing gas 24 from gas supply 12 to a deple¬ tion region 13 located adjacent to and upstream from quenching region 14. The reducing gas reacts exother- mically within the depletion region 13, providing a low density reducing atmosphere therewithin. Nozzle 8 is suitably located to direct reducing gas 24 at and around depletion region 13, so that the reducing gas 24 sub¬ stantially floods the depletion region 13. Valve 16 regulates the volume and velocity through nozzle 8. As shown in FIG. 2, gas nozzle 8 is located upstream of quenching region 14 and is directed substantially normal to the direction of movement of the quench surface. Optionally, 'gas nozzle 8 can be located coaxial with casting nozzle 4 as representatively shown in FIG. 3. The term low density reducing atmosphere, as used in the specification and claims hereof, means a reducing atmosphere having a gas density less than 1 gram per liter and preferably, having a gas density of of less than about 0.5 grams per liter. To obtain the desired low density reducing atmo¬ sphere, gas 24 is exothermically reacted to at least about 800 K, and more preferably, is exothermically reacted to at least about 1300 K. In general, hotter reducing gases are preferred because they will have lower densities and will better minimize the formation and entrapment of gas pockets between quench surface 5 and the deposited molten metal. Entrapped gas pockets are undesirable because they produce ribbon surface defects that degrade the surface smoothness. In extreme cases, the gas pockets will cause perforations through strip 6. A very smooth sur¬ face finish is particularly important when winding mag- -10- netic metal strip to form magnetic cores because surface defects reduce the packing factor of the material. The packing factor is the volume fraction of the actual mag¬ netic material in the wound core (the volume of magnetic material divided by the total core volume) and is often expressed in percent. A smooth surface without defects is also important in optimizing the magnetic properties of strip 6 and in minimizing localized stress concentra¬ tions that would otherwise reduce the mechanical strength of the strip. Gas pockets also insulate the deposited molten metal from quench surface 5 and reduce the quench rate in localized areas. The resultant, non-uniform quench¬ ing produces non-uniform physical properties in strip 6, such as non-uniform strength, ductility and magnetic properties. For example, when casting amorphous metal strip, gas pockets can allow undesired crystallization in localized portions of the strip'. The gas pockets and the local crystallizations produce discontinuities which inhibit mobility of magnetic domain walls, thereby degrading the magnetic properties of the material. Thus, by reducing the entrapment of gas pockets, the invention produces high quality metal strip with improved surface finish and improved physical proper¬ ties. For example, metal strip has been produced with packing factors of at least about 80%, and up to about 95%. The mechanism by which gas pockets are reduced can be more readily explained with reference to FIG. 6. The gas boundary layer velocity profile near quench surface 5 and upstream of melt puddle 18 is shown schematically at 20. The maximum gas boundary layer velocity occurs immediately adjacent to quench surface 5 (substrate) and is equal to the velocity of the moving quench surface. Thus, moving quench surface 5 ordinarily draws cool air from the ambient atmosphere into depletion region 13 and into quenching region 14, the region of the quench sur- 6/00758 -11- face upon which molten metal is deposited. Because of the drafting of relatively cool air into the quenching region, the presence of the hot casting nozzle and the molten metal do not sufficiently heat the local atmo- sphere to significantly reduce the density thereof. Melt puddle 18 wets the substrate surface to an extent determined by various factors including the metal alloy composition, the substrate composition, and the presence of surface films. The pressure exerted by the gas boundary layer at the melt-substrate interface, how¬ ever, acts to locally separate the melt from the sub¬ strate and form entrained gas pockets which will appear as "lift-off" areas 44 on the ribbon underside. The stagnation pressure, P g , of the gas boundary layer (pressure if the layer hit a rigid wall) is given by the formula P s = 2 v 2 where: = gas density, v = sub¬ strate velocity. Therefore, the reduction of gas bound¬ ary layer density or substrate velocity are important in the reduction of the size and the number of gas pockets entrained under the molten metal puddle. For example, removal of the gas boundary layer by casting in vacuum can totally eliminate the lift-off areas in the strip underside. Alternatively, a low density gas in the boundary layer could be employed. The selection of a low molecular weight gas (such as helium) is one way to reduce boundary layer gas density. However, the variety of low molecular weight gases which can be safely and economically used in this fashion is quite limited. The invention provides an economical, safe means for reduc- ing the boundary layer gas density. In accordance with the invention, the boundary layer gas density is reduced by exothermically reacting a reducing gas. As the exo¬ thermic reaction of the reducing gas proceeds, heat pro¬ vided by the reaction causes the density of the gas to diminish as the inverse of the absolute temperature. By exothermically reacting a reducing gas in depletion region 13 at the upstream side of the melt puddle 18, the size and the number of entrained gas pockets under the melt puddle can be substantially reduced. It is important, however, to regulate pertinent factors, such as the composition of the hot, low-density 5 atmosphere, and the parameters of quench surface 5, to substantially prevent the formation of any solid or liquid matter which could precipitate onto quench sur¬ face 5. Such precipitate, if entrained between the melt puddle and quench surface, could produce surface defects 10 and degrade the strip quality. Surprisingly, heat produced by the low density reducing gas atmosphere located proximate to quenching region 14 does not degrade the quenching of the molten metal. Rather, heat produced by the reduction reaction 5 actually improves the uniformity of the quench rate by minimizing the presence of insulating, entrapped gas pockets, and thereby improves the quality of the cast strip. Suitable reducing gases include carbon monoxide * gas and gas mixtures therewith. « 0 The presence of a reducing atmosphere at quench surface 5 has distinct advantages. In particular, a reducing atmosphere minimizes the oxidation of strip 6. In addition, the reducing atmosphere starves quench surface 5 of oxygen and minimizes the oxidation there- 5 of. The decreased oxidation improves the wettability of the quench surface and allows molten metal to be more uniformly deposited on quench surface 5. In the case of a copper base materials in quench surface 5, the decreased oxidation renders the quench surface much more 0 resistant to thermally induced fatigue crack nucleation and growth. The reducing atmosphere also depletes oxy¬ gen frcm the region of nozzle 4 thereby reducing the clogging of nozzle orifice 22, particularly clogging due to oxide particulates. Optionally, additional gas noz- 5 zle 32 may be employed to provide additional reducing gas atmospheres along selected portions of strip 6, as representatively shown in FIG. 2. _ 13 _ FIG. 4 shows an embodiment of the invention wherein the reducing gas is capable of being ignited and burned to form a reducing flame atmosphere. Nozzle 4 deposits molten metal onto quench surface 5 of rotating casting wheel 1 to form strip 6. The depletion means in this embodiment is comprised of gas supply 12, gas nozzle 8 and ignition means 30. Valve 16 regulates the volume and velocity of gas delivered through gas nozzle 8, and a wiper brush 42 conditions quench surface 5 to help reduce oxidation thereon. After gas 24 has mixed with sufficient oxygen, ignition means 30 ignites the gas to produce a heated, low-density reducing atmosphere around depletion region 13 and around quench surface region 14 where molten metal is deposited. Suitable ignition means include spark ignition, hot filament, hot plates and the like. For example, in the embodiment shown in FIG. 4, the hot casting nozzle serves as a suitable ignition means which automatically ignites the reducing gas upon contact therewith. * The resultant flame atmosphere forms a flame plume 28 which begins upstream of quenching region 14 and con¬ sumes oxygen therefrom. In addition, unburned reducing gas within the plume reacts to reduce the oxides on quench surface 5, nozzle 4 and strip 6. The visibility of flame 28 allows easy optimization and control of the gas flow, and plume 28 is effectively drawn around the contour of wheel 1 by the wheel rotation to provide an extended reducing flame atmosphere. As a result, a hot reducing atmosphere is located around quenching surface 14 and for a discrete distant thereafter. The extended flame plume advantageously provides a non-oxidizing, protective atmosphere around strip 6 while it is cool¬ ing. Optionally, additional gas nozzles 32 and ignition means 34 can be employed to provide additional reducing flame plumes 36 along selected portions of strip 6 to further protect the strip from oxidation. A further advantage provided by the hot, reducing flame plume is that the smoothness of the free surface side of the strip (the side not in contact with the quench surface) is significantly improved. Experiments have shown that the mean roughness of the rapidly solidified metal strip, as measured by standard techniques such as pack factor, is significantly reduced when the strip is pro¬ duced in the reducing flame plume of the invention. Proper selection of the reducing gas is impor¬ tant. The combustion product of the burned gas should not produce a liquid or solid phase which could precipi- tate onto quench surface 5 or nozzle 4. For example, hydrogen gas has been unsatisfactory under ordinary con¬ ditions because the combustion product is water which condenr.es onto quench surface 5. As a result, the hydrogen flame plume does not adequately decrease the formation of gas pockets on the quench surface side of strip 6. Therefore, the reducing gas 24 is preferably a gas that will not only burn and consume oxygen in a strongly 'exothermic reaction,,but will als"o produce combustion products that- will remain gaseous at casting condi¬ tions. Carbon monoxide (CO) gas is a preferred gas that satisfies the above criteria, and also provides a desir¬ able, anhydrous, reducing atmosphere. A reducing flame atmosphere provides an efficient means for heating the atmosphere located proximate to melt puddle 18 to very high temperatures, in the order of 1300 - 1500 K. Such temperatures provide very low gas densities around the melt puddle 18. The high tem¬ peratures also increase the kinetics of the reduction reaction to further minimize the oxidation of quench surface 5, nozzle 4 and strip 6. The presence of a hot reducing flame at nozzle 4 also reduces thermal gradi¬ ents therein which might crack the nozzle. Thus, the embodiment of the invention employing a reducing flame atmosphere more efficiently produces a heated, low-density reducing atmosphere around quench surface 5 which improves the smoothness of both sides of the cast strip and more effectively prevents oxidation of quench surface 5, strip 6 and casting nozzle 4. In a particular aspect of the invention, gas supply means 12 produces an initial gas mixture prior to igni- tion which consists essentially of carbon monoxide and oxygen gases. Ignition means 30 ignites the gas to create an exothermic reaction, as representatively shown in FIG. 4. This reaction produces high temperatures and develops a thermally induced, low density, reducing flame atmosphere at depletion region 13, which is located substantially adjacent to and upstream from a quenching region 14 on the surface of the moveable chill body provided by casting wheel 1. Control means, such as the combination of temperature sensor 50 and regula- tor 52 connected to valve 16, controls the initial gas mixture to produce an adjusted reducing flame atmosphere at depletion region 13 and at quenching region 14. This adjusted reducing flame atmosphere has a burnt gas co - position that contains substantially no free oxygen; the burnt gas in flame 28 is substantially free of unre- acted, uncombined oxygen. An initial gas mixture composed of carbon monoxide and oxygen can produce flame temperatures of over 2600°C and can, therefore, produce a very low gas density at depletion region 13 and casting region 14. These high flame temperatures, however, can cause disassociation of molecular O2 into ionic 0, which is highly reactive. As a result, the volume percentage of carbon monoxide in the initial gas mixture should be at least 4 times the volume percentage of oxygen in that mixture to provide a desired degree of effectiveness. In a further aspect of the invention, gas supply means 12 produces an initial gas mixture consisting essentially of carbon monoxide, oxygen and non-reactive diluent gases. For example, gas supply means 12 can provide a selected volume flow rate of CO gas from delivery means 8, which mixes with the ambient air to provide an initial gas mixture that consists essentially / 0758 -16- of CO, 0 2 and N 2 . The presence of the diluent gases advantageously lowers the flame temperature and reduces the disassociation of molecular 0 2 into the highly reac¬ tive O-ion. As a result, the volume percent (vol. %) of carbon monoxide compared to the vol. % of oxygen can be lowered to approach the stoichiometric 2 to 1 ratio, while still producing a desired chemistry in the reduc¬ ing flame atmosphere around the cast strip. Preferably, the vol. % of CO in the initial gas mixture is at least about 2.5 times the vol. % of 0 2 contained in the ini¬ tial mixture. As mentioned previously, a very convenient method for producing the desired initial gas mixture is to mix CO with ambient air to produce a mixture composition consisting essentially of CO, 0 2 and N 2 . With this embodiment of the invention, the amount of CO in the initial gas mixture ranges from about 38-70 vol. %. The lower limit of the range ensures that the resultant flame atmosphere has an optimized, reducing character % and contains substantially no free oxygen. The upper limit of the range ensures that the flame atmosphere does not extinguish. Since the chemistry of the gases in the flame atmosphere is important for optimizing the quality of the cast strip, it is important to accurately monitor the flame chemistry. Direct measurement of the flame composition, however, can be difficult. The present invention advantageously provides an effective control means for effectively monitoring the flame chemistry, which includes a temperature sensor, such as the thermocouple 50 representatively shown in FIG. 4. The control means also includes an adjustment means 52, which, for example, adjusts valve 16 to increase or decrease a flow of CO from gas supply 12 as required. A desired casting regime can be developed by monitoring the change in the flame temperature as a function of the amount of CO provided into the initial gas mixture. In particular, thermocouple 50 senses and _ 17 _ monitors the flame temperature to determine a CO flow rate at which a further, relative increase in the vol. % of carbon monoxide supplied within the initial gas mix¬ ture produces a corresponding, relative decrease in the flame temperature. From the presence of such condi¬ tions, one can reliably infer the establishment of the desired casting regime; a regime in which the hot flame atmosphere is substantially free of unreacted oxygen. Rapid quenching under conditions such as those above can produce a metastable, homogeneous, ductile material. The metastable material may be glassy, in which case there is no long range order. X-ray diffrac¬ tion patterns of glassy metal alloys show only a diffuse halo, similar to that observed for inorganic oxide glasses. Such glassy alloys must be at least 50% glassy to be sufficiently ductile to permit subsequent handl¬ ing, such as stamping complex shape from ribbons of the alloys. Preferably, the glassy metal alloys must be at least 80% glassy, and most preferably substantially (or totally) glassy, to attain superior ductility. The metastable phase may also be a solid solution of the constituent elements. In the case of the alloys of the invention, such metastable, solid solution phases are not ordinarily produced under conventional process- ing techniques employed in the art of fabricating crystalline alloys. X-ray diffraction patterns of the solid solution alloys show the sharp diffraction peaks characteristic of crystalline alloys, with some broaden¬ ing of the peaks due to desired fine-grained size of crystallites. Such metastable materials are also duc¬ tile when produced under the conditions described above. Rapid quenching under conditions such as those above can also produce an equilibrium microcrystalline alloy. The term microcrystalline alloy, as used herein, menas an alloy which, upon rapid solidification, has a grain size less than about 10 micrometers (0.004 in.). Preferably such an alloy has a grain size ranging from about 100 nanometers (0.000004 in.) to 10 micrometers (0.0004 in.), and most preferably from about 1 micrometer (0.00004 in.) to 5 micrometers (0.0002 in.). Microcrystalline alloys are formed by cooling a melt of the desired composition at a rate of at least ~ about 10 °C./sec, and preferably at least about 10 5o C/sec. A variety of rapid quenching techniques, well known to the microcrystalline alloy art, are available for producing microcrystalline powders, wires, ribbon and sheet. Typically, a particular composition is selected, powders or granules of the requisite elements in the desired portions are melted and homogenized, and the molten alloy is rapidly quenched on a chill surface, such as a rapidly rotating cylinder, or in a suitable fluid medium, such as water. The material of the invention is advantageously produced in foil (or ribbon) form, and may be used in product applications as cast, whether the material is glassy or a solid solution. Alternatively, foils of glassy metal alloys may be heat treated to obtain a crystalline phase, preferably fine-grained, in order to promote longer die life when stamping complex shapes. As shown in FIG. 5, the invention may optionally include a flexible hugger belt 38 which entrains strip 6 against quench surface 5 to prolong cooling contact therewith. The prolonged contact improves the quenching of strip 6 by providing a more uniform and prolonged cooling period for the strip. Guide wheels 40 position belt 38 in the desired hugging position along quench surface 5, and a drive means moves belt 38 such that the belt portion in hugging relation to quench surface 5 moves at a velocity substantially equal to the velocity of the quench surface. Preferably, belt 38 overlaps the marginal portions of strip 6 to directly contact and frictionally engage quench surface 5. This frictional -engagement provides the required driving means to move the belt. Considerable effort has been expended to develop devices and procedures for forming thicker strips of —ιy— rapidly solidified metal because such strip can more easily be used as a direct substitute for materials presently employed in existing commercial applica¬ tions. Since the present invention significantly improves the contact between the stream of molten metal and the chilled quench surface, there is improved heat transport away from the molten metal. The improved heat transport, in turn, provides a more uniform and more rapid solidification of the molten metal to produce a higher quality thick strip, i.e. strip having a thick¬ ness ranging from about 15 micrometers to as high as about 70 micrometers and more. Similarly, considerable effort has been expended to form thinner strips of rapidly solidified metal. Very thin metal strip, less than about 15 microns and pref¬ erably about 8 microns in thickness, is highly desirable in various commercial applications. In brazing applica¬ tions, for example, the filler metals used in brazed .j-oint normally ha^ inferior mechanical properties com- pared to the base metals. To optimize the mechanical properties of a brazed assembly, the brazed joint is made very thin. Thus, when filler material in foil form is placed directly in the joint area prior to the braz¬ ing operation, the joint strength can be optimized by using a very thin brazing foil. In magnetic applications with high frequency elec¬ tronics (over 10 kHz), power losses in magnetic devices are proportional to the thickness (t) of the magnetic materials. In other magnetic applications such as satu- rable reactors, power losses are proportional to the thickness dimension of the magnetic material raised to the second power (t ώ ) when the material is saturated rapidly. Thus, thin ribbon decreases the power losses in the reactor. In addition, thin ribbon requires less time to saturate; as a result, shorter and sharper out¬ put pulses can be obtained from the reactor. Also, thin ribbons decrease the induced voltage per lamination and therefore, require less insulation between the lamina¬ tions. In inductors for linear induction accelerators, losses are again related to t , and the thinner ribbon will reduce power losses. Also, thin ribbon saturates more easily and rapidly and can be used to produce shorter pulse accelerators. In addition, the thinner ribbon will require reduced insulation between the lami¬ nations. A further advantage of thin strip is that the strip experiences less bending stresses when wound to a given diameter. Excessive bending stresses will degrade the magnetic properties through the phenomenon of magneto¬ striction. The apparatus and method of the invention are par¬ ticularly useful for forming very thin metal strip. Since the invention significantly reduces the size and depth of gas pocket defects, there is less chance that such a defect will be large enough to perforate the cast* strip. As a result, very thin strip can be cast because there is less probability that a defect large enough to perforate the strip will form. Thus, the invention can be adapted to cast very thin metal strip, which as-cast, is less than about 15 micrometers thick. Preferably, the cast strip has a thickness of 12 micrometers or less. More preferably, the cast strip thickness ranges from 7 to 12 micrometers. In addition, the thin metal strip has a width dimension which measures at least about 1.5 millimeters, and preferably measures at least about 10 mm. The following examples are presented to provide a more complete understanding of the invention. The spe¬ cific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles of the invention are exemplary and should not be construed as limiting the scope of the invention. Alloy chemis¬ tries are expressed as nominal compositions with sub¬ scripts in atom percent. EXAMPLE 1 A forced-convection-cooled, casting wheel having a plain carbon steel substrate was used to prepare nickel- base and iron-base glassy metal ribbons. The casting wheel had an internal cooling structure similar to that described in U.S. Patent 4,307,771, a diameter of 38 cm and a width of 5 cm. It was rotated at a speed of 890 rpm, corresponding to a circumferential surface velocity of 18 m/s. The substrate was conditioned continuously during the run by an idling brush wheel inclined about 10° out of the casting direction. A nozzle having a slotted orifice of 0.4 millimeter width and 25 milli¬ meter length defined by a first lip and a second lip each having a width of 1.5 millimeters (lips numbered in direction of rotation of the chill roll) was mounted perpendicular to the direction of movement of the peri¬ pheral surface of the casting wheel, such that the gap between the second lip and the gap between the first lip and-the surface of the casting wheel was .20 milli- meter. Nickel-base metal alloy having composition NiggCr-yFe-B-,^Sig (subscripts in atomic percent) with a melting point of about 1000°C was supplied to the nozzle from a pressurized crucible, the metal within the cruci¬ ble being maintained under pressure of about 3.5 psig (24 kPa) at temperature of 1300°C. Pressure was sup¬ plied by means of an argon blanket. The molten metal was expelled through the slotted orifice at the rate of 6.6 kilograms per minute. It solidified on the surface of the chill roll into a strip of 0.033 millimeter thickness having width of 2.54 cm. Upon examination using X-ray diffractometry, the strip was found to be amorphous in structure. The ribbon showed significant populations of entrapped air pockets in the underside. A dark oxidation track formed on the substrate surface during ribbon casting, limiting the ribbon substrate adhesion. EXAMPLE 2 The procedure of Example 1 was repeated, employing the equipment, process conditions, metal and alloys used in Example 1 except that a carbon monoxide flame was directed at the ribbon casting track upstream of the melt puddle to reduce oxidation and promote ribbon- substrate adhesion. The combined actions of the flame and the conditioning brush reduced the substrate oxida¬ tion, increased adhesion and produced ribbon having good geometric uniformity. The best results were obtained when the distance between the carbon monoxide flame and the back of the melt puddle was less than about 2 cm (<1 inch). Tensile specimens cut from the strip in longitu¬ dinal and transverse direction exhibited equal tensile strength and elongation. The strip had substantially isotropic tensile properties. EXAMPLE 3 The. procedure of Example 1 was repeated, employing the equipment, process conditions metal and alloy sum- marized in the Table I below to obtain the product described therein. TABLE I Alloy (At.%) Fe 81 B 13 # 5 C 2 S_ 3 > 5 Casting wheel diameter (am) 38 5 Casting wheel width (am) 5 Casting wheel rpn 890 Nozzle orifice width (mm) 2.5 Nozzle orifice length (mm) 0.4 width first lip (mm) 1.5 Width second lip (mm) 1.5 Gap-second lip to casting wheel (mm) 0.20 Gap-first lip to casting wheel (mm) 0.20 Melting point of metal ( °C) 1150 Pressure applied to crucible (kPa) 24 Temp, of metal in crucible approx. ( °C) 1350 Thickness of strip (mm) .02 Width of strip (mm) 25 Structure of strip Anorphous The iron-base ribbon was annealed in an inert gas atmo¬ sphere for 2 hours at a temperature of 365 °C in a f ield of 80 amperes/meter applied longitudinal of the ribbon length. _ _ A photomicrograph showing the underside of the iron-base, amorphous ribbon is depicted in Fig. 7. Note that the included air pockets shown are rather large and elongated. EXAMPLE 4 The procedure of Example 3 was repeated employing the same equipment, process conditions and alloy except that a carbon monoxide flame was directed at the ribbon casting track upstream of the melt puddle to reduce oxi- dation and promote ribbon substrate adhesion. A photo¬ micrograph showing the underside of the iron-base amor¬ phous ribbon produced using the carbon monoxide flame is depicted in Fig. 8. Note the significant reduction in included air pockets on the underside of iron-base rib- bon cast using the carbon monoxide flame as compared with those shown in Fig. 7. Magnetic properties of the ferromagnetic ribbons as well as the pack factor thereof were also improved. Similar improvements in the under- side of nickel-base amorphous ribbon have also been observed. Thus, experiments have shown remarkable improvement of ribbon surface smoothness, luster, and ductility over material cast in a conventional manner. While the intrinsic wetting of a copper substrate by ferrous melts may not be as great as the wetting of an iron-based sub¬ strate, the use of a carbon monoxide flame enhances melt-copper substrate wetting to the point where a cop¬ per substrate is a viable material for the production of high quality, defect-free strip. Such a defect-free casting capability allows the production of very thin ribbon (on the order of about 7 micrometers thick). Additionally, the improved melt-substrate contact pro¬ vided by carbon monoxide flame-assisted casting improves overall quench rate and enables the production of a given ribbon composition at a thickness greater than usual. EXAMPLE 5 Adiabatic (maximum) flame temperatures were cal¬ culated and compared with limited experimental measure¬ ments. Oxygen molecule and ion concentrations were calculated because of the chemically active nature of these gas species. Predetermined CO-0 2 and CO-air compositions were produced by flowing commercial purity gasses through the mixing head of a torch assembly. Each of the premixed gasses was delivered into a 12 mm inside diameter clear fused quartz combustion tube under 35 kPa (5 psi) pres¬ sure and up to 500 cc/sec flow rate. A moveable Pt/Pt- 13Rh (R type) thermocouple, made of 0.5 mm diameter wire, was used in conjunction with a Fluke 2160A digital thermometer with analog output to measure flame tempera¬ ture. Maximum flame temperature for each premixed gas composition was measured and recorded by carefully scan¬ ning the gas reaction zone inside the combustion tube with the thermocouple. Sources of heat loss such,, as radiation, conduction through thermocouple leads, etc. were not considered. In another kind of measurement, the thermocouple was traversed at about 4 cm/sec through the diameter of an unconstrained CO flame in air. In addition to providing flame temperature, the resultant thermal profiles were also used to calculate local flame chemistries. FIG. 9 is a schematic representation of calculated, C0-0 2 flame, burnt gas thermochemistries as a function of the relative amount of CO in the initial gas mixture composition. "M" designates experimentally measured, maximum flame temperature data points. As representa¬ tively shown in FIG. 9, the burnt gas composition in the flame contains an approximately zero amount of free, unreacted oxygen (0 2 ,o) when the initial gas mixture contains at least about 80 vol. % of CO. To sustain a burning flame, however, the amount of CO in the initial gas mixture should be less than about 95 vol. %, and preferably less than about 92 vol. %. FIG. 9 also representatively shows that the condition of a substan¬ tially zero amount of free oxygen in the burnt gas cor¬ responds approximately with the regime in which an incremental increase of the vol. % CO within the initial gas mixture produces an incremental decrease in the flame temperature. The graph in FIG. 10 is a schematic representation of calculated, CO-air flame, burnt gas thermochemistries as a function , of the relative amount of CO in the ini- tial gas mixture. "M" again designates experimentally measured data points of maximum flame temperature. With the CO-air flame, the burnt gas composition of the flame contains a substantially zero amount of free oxygen when the initial gas mixture contains about 38-70 vol. % CO. Above about 70 vol. % CO in the initial gas mix¬ ture, the flame extinguishes. Again the condition of a substantially zero amount of uncombined oxygen in the burnt gas approximately corresponds to the regime in which an incremental increase in the vol. % of CO in the initial gas mixture produces an incremental decrease in the flame temperature. EXAMPLE 6 A Fe 7 gB-, 3 Si- alloy was cast into amorphous strip form on a 38 cm diameter beryllium-copper chill wheel rotated to provide a quench surface speed of about 20 m/sec. The melt temperature was about 1623 K and the casting pressure on the melt was about 19 kPa. The casting nozzle had a slot orifice which measured about 0.38 mm in width and about 5 cm in length. The nozzle was offset from the top-dead-center of the chill wheel by approximately 1.6 mm in the downstream direction was was positioned to provide a casting gap of about 0.15 mm between the nozzle orifice and quench surface. Experimental runs were made employing two different CO-flame chemistries. One CO-flame (CO flow rate of 22 cc/sec) contained excessive free oxygen (column 1), and the other CO-flame (CO flow rate of 38 cc/sec) contained substantially zero free oxygen (column 2). Representative power loss data and excitation power data for two resultant cast strips are shown in TABLE 2. From a comparison of the data, it is readily apparent that the optimized CO-flame containing no free oxygen in the burnt gas chemistry significantly reduced the power loss and excitation power. TABLE 2 Power loss (W/kg) @ 1.3 T 0 .170 0.119 1.4 T 0.211 0.138 Excitation Power (VA/kg) @ 1.3 T 0.232 0.196 1.4 T 0.402 0.252 EXAMPLE 7 A Fe-B-Si-C alloy was rapidly solidified into amor- phous strip form on a 38 cm diameter beryllium-copper chill wheel, which was rotated to provide a peripheral quench surface speed of about 18 m/sec. The melt tem¬ perature was approximately 1623 K and the casting pres¬ sure was about 24 kPa. The slot orifice ' of the casting nozzle measured about 0.38 cm wide and about 5 cm long. The nozzle was offset downstream from the wheel top-dead-center by about 3.2 mm, and the casting gap was about 0.13 mm. In experimental runs, strip was cast employing three different sets of conditions. Under the first set of conditions, the alloy was cast in the low tempera¬ ture, ambient air with no flame (column 1). Under the second set of conditions, the alloy was cast in a burn¬ ing CO-flame that contained substantially zero free oxy- gen in the burnt gas (column 2) . Under the third set of conditions, the alloy was cast in a very hot CO-flame that contained excess oxygen (column 3). Certain char¬ acteristics of the resultant cast strips are summarized in Table 3. Frcm a comparison of the data, it is apparent that in the very hot temperatures produced by a CO-flame con¬ taining excess oxygen, the magnetic characteristics of the cast strip were degraded. The strip cast in the CO- flame containing no free oxygen in the burnt gas had the best magnetic properties. TABLE 3 (CO + (air) (CO) excess 0-) 1 2 3 Pack Factor 70% 83% 72% max x 10 3 129 176 83 Br (Tesla) 1.24 1.46 0.94 E λ (Tesla) 1.47 1.56 1.32 Br/B x 0.84 0.94 0.71 Power Loss (w/kg) 1.4 T 0.25 0.24 0.29 Excitation Power (VAAg) 1.4 T 0.70 0.28 2.47 Avg. Thickness (micrometer) 16 16 20 Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope, of the invention ' as defined by the subjoined claims.

Claims: -28- I CLAIM : 1. An apparatus for casting metal strip, compris¬ ing: a. a moveable chill body having a quench sur- face thereon; b. nozzle means for depositing a stream of molten metal on a quenching region of said surface to form said strip; c. gas supply means for providing an initial gas mixture, which consists essentially of carbon monoxide and oxygen gases; d. ignition means for igniting said initial gas mixture to create an exothermic reaction which provides a low density, reducing flame atmosphere at a depletion region located substantially adjacent to and upstream from said quenching region; and e. control means for controlling said initial gas mixture to produce an adjusted reducing flame atmo- sphere at said depletion region, said adjusted reducing flame having a burnt gas composition which includes sub¬ stantially no free oxygen. 2. An apparatus as recited in claim 1, wherein said controlled initial gas mixture contains a volume percentage of carbon monoxide which is at least about 4 times the volume percentage of oxygen contained therein. 3. An apparatus as recited in claim 1, wherein said initial gas mixture consists essentially of carbon monoxide, oxygen and one or more non-reactive diluent gases. 4. An apparatus as recited in claim 3, wherein said non-reactive diluent gas is composed of nitrogen gas. 5. An apparatus as recited in claim 3, wherein said controlled initial gas mixture contains a volume percentage of carbon monoxide which is at least about 2.5 times the volume percentage of oxygen contained therein. _ _ 6. An apparatus as recited in claim 3, wherein said control means provides an initial gas mixture con¬ sisting essentially of 38-70 vol. % carbon monoxide in a mixture with ambient air. 7. An apparatus as recited in claim 1, wherein said control means comprises: a. temperature sensing means for sensing flame temperature; and b. adjustment means for adjusting said control means to provide a casting regime in which a relative increase in the volume percent of carbon monoxide sup¬ plied within said initial gas mixture produces a cor¬ responding, relative decrease in said flame temperature. 8. A method for casting metal strip, comprising the steps of: a. moving a chill body having a quench surface thereon; b. depositing a stream of molten metal on a quenchrrig region of said surface to fo * rm said strip; c. supplying an initial gas mixture consisting essentially of carbon monoxide and oxygen; d. igniting said initial gas mixture to create an exothermic reaction which provides a low density, reducing flame atmosphere at a depletion region located substantially adjacent to and upstream frαn said quench¬ ing region; and e. controlling said initial gas mixture to pro¬ duce an adjusted reducing flame atmosphere, which con¬ tains substantially zero free oxygen, at said depletion region. 9 . A method as recited in claim 8 , wherein said controll ing step ( e ) provides an init ial gas m ixture which cons ists essent ial ly of carbon monoxide , oxygen and non-reactive diluent gases . 10 . A method as recited in cla im 9. wherein said controlling step (e ) provides an initial gas mixture with a compos it ion cons isting essentially of about 38-70 vol . % carbon monoxide m ixed with ambient a ir . 11. A method as recited in claim 8, wherein said controlling step (e) comprises the steps of: a. sensing the temperature of said flame; and b. adjusting the amount of carbon monoxide in said initial gas mixture to provide an operating regime in which a relative increase in the volume percent of carbon monoxide within said initial gas mixture produces a corresponding, relative decrease in said flame tem¬ perature.

Inventors: LIEBERMANN HOWARD HORST, LIEBERMANN, HOWARD, HORST
Applicants: ALLIED CORP, ALLIED CORPORATION
Application year: 1986
Main classification: B22D11
IPC codes: B22D11