|Title of Invention||
"EQUIAXED FINE GRAIN QUENCH SURFACE"
|Abstract||1. A quench surface for rapid solidification of molten alloy into strip having a microcrystalline or amorphous structure, said quench surface having composed of a thermally conducting alloy having a microstructure consisting of fine, equiaxed, recrystallized grains, the average size of said grains being less than 200 um and none of said grains being larger than 500 um, said grains having a iight Gaussian grain size distribution.|
|Full Text||EQULAXED FINE GRAIN QUENCH SURFACE BACKGROUND OF THE INVENTION
1. Field Of The Invention
This invention relates to manufacture of ribbon or wire by rapid quenching of a molten alloy; and more particularly, to characteristics of the surface used to obtain the rapid quench. A quench surface having a fine, equiaxed, recrystallized microstructure. exhibiting a tight Gaussian grain size distributor, has surprisingly been found to improve the quality of the surface finish of the rapidly solidified strip
2. Description Of The Prior Art
Continuous casting of alloy strip is accomplished by depositing molten alloy onto a rotating casting wheel Strip forms as the molten alloy stream is attenuated and solidified by the wheel's moving quench surface. For continuous casting, this quench surface must withstand mechanical damage which may arise from cyclical stressing due to thermal cycling during casting Means by which improved performance of the quench surface can be achieved include the use of alloys having high thermal conductivity and high mechanics.! strength. Examples include copper alloys of various lords, steels and the like Alternatively, various surfaces can be plated onto the casting wheel quench surface to improve its performance, as disclosed in European Patent No EP0024506, A suitable casting procedure is set forth in detail in U.S. Patent 4,' 42,571, the disclosure of which is incorporated herein by reference
Casting wheel quench surfaces of the prior art generally involve one of two forms: monolithic or component Monolithic quench surfaces comprise a solid block of alloy fashionec into the form of a casting wheel that is .optionally provided with, cooling channels Component quench surfaces comprise a plurality of pieces that, when assembled, constitute a casting wheel, as disclosed in U S. Patent No 4,537.2 39 The casting wheel quench surface improvements of the present disclosure are applicable to all kinds of casting wheels
When selecting materials for construction of a casting wheel quench surface, certain mechanical properties such as hardness, tensile and yield strength, and elongation have generally been considered, sometimes in combination with thermal cone activity This was done in an effort to achieve the best combinanon of thermal coneuctivity and mechanical strength properties possible for a given alloy The reason fortus is basically twofold. I) to provide a high quench in the cast, 2) to resist mechanical damage of the quench surface which causes degradation of the strip's geometric definition. Dynamic or cyclical mechanj<:al properties must also be considered in order to develop a quench surface which has superior performance characteristics> One consequence of a poor selection of the material is rapid deterioration of the casting wheel surface due to the formation of pits. Pits are small defects that are usually observed when they larger than about 0.1 mm deep, they grow in depth and diameter as casting proceeds. These surface irregularities, result in corresponding defects, "pips", in the cast ribbon These pips not only affect the surface finish of the ribbon, but can also reduce die ribbon's usefulness m such applications .as transformer cones, antitheft systems and brazed articles. The importance of these surface defects to tie value of the rapidly quenched ribbon and the customer's satisfaction is evident
The surface defects limit the life of the c;*sang wheel quench surface and reduces the surface quality of ribbon cast thereon This, in turn, reduces the usefulness of such ribbon 1o the customer, whose designs must account for properties associated with the wor^t surface quality of the ribbon he might receive Even when a good selection of mechanical and thermal properties is made, as is the case with the Cu Cr and Cu Be type alloys, the deterioration of a casting wheel's quench surface finish progresses rapidly. There exists a need in the art for a quench surface that resists rapid deterioration and produces, for a prolonged period of time, ribbon having a surface which is defect free.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for continuous casting of alloy strip Generally stated, the apparatus has a casting wheel comprising a rapidly moving querch surface that cools a molten alloy layer deposited thereon for rapid solidification into a continuous alloy strip The quench surface is composed of a thermally conecting alloy having a fine, equaxed, recrystailized microstructure, exhibiting a tight Gaussian grain size distribution.
The casting wheel of the present invention optionally has a cooling means for maintaining said quench surface ar a substantially constant temperature throughout the time that molten alloy is deposite and quenched thereon. A nozzle is mounted in spac ed relationship to the quench surface for exp«llmg molten alloy therefrom The molfen alloy is directed by the nozzle to a region of the quench surface, whereon it is deposited A res,ervoir in communication with the nozzle holds a supply of molten alloy and feeds it to the nozzle.
Preferably, the quench surface is comprised of fine equiaxed recrystallized grains exhibiting a tight Gaussian grain size distribution and an average gram size less that 80 µm Use of a quench surface having these qualities significantly increases the service life of the quench surface Run times for casts conducted on the quench surface are significantly lengthened, and the quantity of material cast during each run is increased by a factor as high as three or more. Ribbon cast on the quench surfaces exhibits far fewer surface defects, and hence, an increased pack factor (% lamination), and the efficiencies of electrical power distribution transformers made from such ribbon are improved. Run response of the quench surface during casting is remarkably consistent from one cast to another, with the result that the run times of substantially the same duration are repeatable and scheduling of maintenance is facilitated. Advantageously, yields of ribbon rapidly solidified on such surfaces are marketly improved, maintenance of the surfaces is minimized, and the reliability of the process is increased.
BRIEF DESCRIPTION OF TOE DRAWINGS
The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description and the accompanying drawings, in which
Fig 1 is a. perspective view of an apparatus for continuous casting of metallic strip,
Fig 2 illustrates the effect of the bimodal grain size distribution (quantified by the % area of large grains) on the life of hot forged casting wheels having conventional quench surfaces,
Fig 3 is the grain size distribution of "good" and '"bad" hot forged wheels, showing the bimodal grain size distribution;
Fig 4 illastrates how the degree of cold work effects the average grain size,
Fig 5 is the grain size distribution obtained by cold working the wheel as described herein;
Fig 6 is a micrograph of a cold forged wheel showing the recrystallized microstructure, the average grain size is less man 30µm. The normalized nbbon quantity cast for "his wheel was 2 9.
Fig. 7 is a micrograph of a hot forged wheel, the average grain size is less than 30µm. The normalized ribbon quantity cast for mis wheel was 1 7
Fig. 8 is a micrograph of a cold forged and aged wheel, the average grain size is less than 30µm. The normalized ribbon quantity cast for this wheel was C 3
Fig. 9 is a grain size distribution obtained by extrusion, showing a tight Gaussian grain size distribution;
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term 'amorphous metallic alloys" means a metallic alloy that substannally lacks any long range order and is characterized by X-ray diffraction intensity maxima which are qualitatively similar to those observed for liquids or inorganic oxide glasses
The term microcrystalline alloy, as used herein, means an alloy that has .1 gram size less than 10 µm (0 0004 in.) Preferably such an alloy has a grain size ranging from about 100 run (0 000004 in ) to 10 µm (0 00C4 in.), and most preferably from about lµm (0 00004 in ) to 5 µm (0.0002 in.)
Grain size as used herein is taken to have been determined by an image analyzer looking directly at an alloy sample that has been polished and correctly etched to reveal grain boundaries The average grain size was determined using five different locations within the sample chosen at random In all cases the magnification was reduced to that at which the largest grains in the sample fit completely within the field of view. If there were any uncertainties, the grain size was determined at different magnifications to ensure it did not change with magnification
As used herein, the term "strip" means a slender body, the, transverse dimensions of which are much smaller than its length Strip thus includes wire, ribbon, and sheet, all of regular or irregular cross-section.
The term "rapid solidification", as used herein throughout the specification and claims, refers to cooling of a melt at a rate of at least about 104 to 106 C/s. A variety of rapid solidification technique:; are available for fabricating strip within the scope of the present invention such as, for example, spray depositing onto a chilled surface, jet casting, planar flow casting, etc
As used herein, the tern t "wheel" means a body having a substantially circular cross section raving a width (in the axial direction) which is smaller than its diameter In contrast, a roller is generally understood to have a greater width man diameter
The term "thermally conducting" as used herein, means that the quench surface has a thermal conductivity value greater man 40 W/m K and less than about
400 W/m K, and more preferably greater than 60 W/m K and less than about 400 W-'m K, and most preferably greater than 80 W/m K and less man 400 W/m K.
As used herein the term "normalized ribbon quantity cast7' refers to the quartity/mass of ribbon that it was possible to cast on a particular wheel, normalized to a standard wheel.
The term "solution heat treatment", as used herein, means heating the alloy to a temperature at which all the alloy additions are in solution. This often results in recystallization occurring once the alloy additions are in solution The actual solution heat treatment temperature depends upon the alloy Copper beryllium alloy 25 is usually solution treated within the range 745 to 8 t0°C After solution heat treatment the alloy is rapidly cooled to maintain the alloy additions in solution. In this state, the alloy is soft and ductile and easily worked.
As used herein the term "aging" means the low temperature exposure used to precipitate alloy additions from the solution heat treated alloy. The precipitation of strengthening phases hardens the alloy. Aging times and temperature are optimized to obtain the maximum hardness and, hence, strength. The copper beryllium alloy 25 is usually aged at 260 to 370°C for 1/2 to 4 hours. Excessive aging tune results in loss of hardness, strength and ductility Because copper beryllium alloys are usually sold in the solution heat treated condition, aging of copper beryllium alloys is usually referred to simply as "heat treatment".
The term "Gaussian ", as used herein, means a normal standard distribution around an average value For certain cases close to zero in the examples the distribution is positively skewed, because the grains can not have negative values Such cases in this work are still referred to for simplicity as a Gaussiar distribution
As used herein the term "tight" means that there is very little variance around the Gaussian or normal distribution The term narrow Gaussian distribution could also be used as opposed to a wide Gaussian distribution.
In this specification and in the appended claims, the apparatus is described with reference to the section of a casting wheel which is located at the wheel's periphery and serves as a quench surface. It will be appreciated that the principles of the invention are
applicable, as wall, to quench surface configurations such as a belt, having shape and structure different from those of a wheel, or to casting wheel configurations in which the section that serves as a quench surface is located on the face of the wheel or another portion of the wheel other than the wheel's periphery
The present invention provides a quench surface for use in rapid solidification, a process for us:ng the quench surface in the rapid solidification of metallic strip, and a process for making the quench surface.
Referring to Fig. 1, there is shown generally at 10, an apparatus for rapid solidification of metallic strip Apparatus 10 has an annular casting wheel 1 rotatably mounted on its longitudinal axis, reservoir 2 for holding molten metal and induction heating coils 3 Reservoir 2 is in communication with slotted nozzle 4, which is mounted in proximity to the surface 5 of annular casting wheel 1. Reservoir 2 is further equipped with means (not shown) for pressurizing the molten metal contained therein to effect expulsion thereof though nozzle 4. In operation, molten metal maintained under pressure in reservoir 2 is ejected through nozzle 4 onto the rapidly moving casting wheel surface 5, whereon it solidifies to form strip 6 After solidification, strip 6 separates from the casting wheel and is flung away therefrom to be collected by a wirder or other suitable collection device (not shown)
The material of which the casting wheel quench surface 5 is comprised may be copper or any other metal or alio;,' having relatively high thermal conductivity. This requirement is particularly applicable if it is desired to make amorphous or metastable strip. Preferred materials of construction for surface 5 include precipitation hardened copper alloys, such as chromium copper or beryllium copper, dispersion hardened alloys, and oxygen-free copper. If desired, the surface 5 may be highly polished or chrome-plated or the like to obtain strip having smooth surface characteristics To provide additional protection against erosion, corrosion or thermal faigue, the surface of me casting wheel may be coated with a suitable resistant or high-melting material Typically, a ceramic coating or a coating of corrosion-resistant, high-melting temperature metal is applicable, provided that the wettability of the molten metal or alloy being cast on the chill surface is adequate.
The deposition of molten alloy onto the quench surface as the wheel rotates during casting results in a large radial thermal gradient near the surface and large thermal cyclic stresses. These effects may combine to mechanically degrade the quench surface during casting.
We have discovered that the problems of mechanical degradation described above can be minimized by the use of a quench surface comprised of fine, equiaxed. recrystallized grains having a tight Gaussian grain size distribution with substantally no grain larger than 500 µm Copper based alloys typically have a bimodal grain size distribution In fact, copper alloys are the only alloys for which the American Society of Testing & Measurement grain size standard, ASTM El 12, permits the average grain size to be specified by two sizes. Of the two size; specified, one size is for the fine grains and one size is for the large grains. Typical values for these sizes would be 100 µm and 600 µm, respectively. For copper alloys, a range in grain sizes of about 5 to 1000 urn is normal.
The large grain size, commonly occurring in copper alloys because of the bimodal grain size distribution, is detrimental to the durability of the casting wheel. A series of copper casting wheels fabricated by hot forging were investigated in detail All had a typical bimodal distribution typified by the ASTM grain size of 20 and 500 µm. It was found possible to quantify the degree of bimodal distribution and to take some account of the large grain size by using an image analyzer to determine the percentage of the casting wheel material with a gran size above 250 µm. As shown in Fig. 2, the hot forged wheels with a high percentage of large grains had a small normalized ribbon quantity cast, while the ones with a small percentage had a much larger normalized ribbon quantity cast. Fig. 3 depicts the grain size distribution of "good" and "bad" wheels. While each of the "good" and "bad" wheels have bimodal distributions, the wheel with the higher normalized quantity cast (1,4 compared to 0.04) has fewer large grains. Clearly large grains and a bimodal grain size distribution are deleterious to quench surface performance in the continuous casting of metal or alloy strip. Under these circumstances, the specific manner in which quench surface degradation occurs is through the formation of very small cracks in the surface hereof
Subsequently deposited molten metal or alloy then enters these small cracks, solidifies therein, and is pulled out, together with adjacent quench surface material, as the cast strip is separated from the quench surface during operation. The degradation process is degenerative, growing progressively worse with time Cracked or pulled out spots on the quench surface are called "puts", while the associated replicated protrusions attached to the underside of the cast strip are called "pips "
It should be beneficial to reduce the bimodal distribution, by reducing the area of large grains further. However, it is difficult to obtain essentially a. 100% fine gram size with conventional hot forging processes Conventional hot forging usually involves working the metal by discrete hammer blows into an annular quench surface, to prepare it for subsequent heat treatment in order to develop high strength The limitation of this mechanical working method is largely its discrete, incremental nature. That is, not all volume elements of the quench surface are equally worked and subsequent bimodal gram size distributions can occur, with the occurrence of large grains in a matrix of fine grains
Alternate fabrication routes were therefore explored These included forward and back extrusion, flow forming and hot and cold forging. Several provided an homogenous fine grained microostructure While some of these improved wheel life. it was surprisingly found mat even with an extremely fine ( Surprisingly, the best results were obtained with techniques that formed fine, equiaxed, recrystallized grains with a tight Gaussian grain size distribution. The benefits of such a microstructure are not limited to longer wheel life, but also include better equipment utilization and the production of ribbon having a superior surface finish. In the case of ribbon made from magnetic alloys, a better surface finish provides a higher packing factor, and a more efficient transformer The benefits associated with
improved ribbon quality have been found to significantly increase once the ribbon is mads effectively "pip" free
The following examples are presented to provide a more complete understanding of the invention The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention art' exemplary and should not be construed as limiting the scope of the invention
An ingot of the copper beryllium alloy 25 was hot side forged at 700°C and pierced, after which it was hot forged and then cold forged to the final, desired casting wheel size. Specifically, the billet was hot forged to an intermediate size and than subjected to a 33% cold reduction to the final wheel size Fig. 4 shows the average gram size obtained for samples given a standard hot forge and then cold forged to varying reductions prior to a standard solution heal treatment The grain size obtained remains constant for a large range of cold work and can be expected to only change slightly outside the immediate range investigated in Fig. 4.
The 30% cold worked casting wheel was then given a standard solution heat treatment and aging prior to machining to the exact wheel dimensions and tolerances The resultant Gaussian grain size distribution is shown in Fig 5 These fine, equiaxed, recrystallized grains, shown in Fig. 6, resulted in this wheel having an extremely long life The wheel described by Figs. 5 & 6 had a normalized ribbon quantity cast of 2 9, which is approximately twice the value of the"best" hot forged wheel described in Fig. 2
In most cases, the ribbon produced using this wheel had no pips As a result its lamination factor was increased The desirability of this ribbon is, therefore, evident
Additional casting wheels were fabricated by the process described above In al cases, the wheel microstructure was comprised of fine, recrystallized, equiaxed grains exhibiting a tight Gaussian grain size distribution. These casting wheels, all demonstrated superior casting performance as measured by the normalized ribbon quantity cast. This information is given in Table 1
* The grain sizes reported in Table 1 were obtained using plastic replica; of the wheel surface, which has the advantage of being a non destructive technique. This technique gives a slightly larger grain size (~ -10um for these microstrctures) than the destructive technique used herein for all the other grain size measurements
An angol of the copper beryllium 25 alloy was hot side forged at 700°C and pierced, as in example 1. In this example, the billet was then hot forged all the way to the final casting wheel size. An homogeneous microstructure was; produced with a very fine average grain size, less than 30um. However, because of the absence of cold work, the grain:; were not all equiaxed, annealing twins were found within the grains and the grain si2;e distribution was; not Gaussian in shape. The microstructure of this wh«:el is shown in Fig. 7. Even though the microstructure was homogeneous and the average grain size was very fine EXAMPLE 3 An inget of the copper beryllium alloy 23 was hot side forged at 700°C and pierced The billet was hot forged to an intermediate size and then given a 30% cold
reduchon to the final wheel size as in Example 1 After the cold work, the material was aged. Unlike the soluuonized and aged matenal of Example 1, a recrystalized micostracture was not produced in this case. Instead, the wheel had a fine homogenous microstructure with highly deformed grams, which had an average grain size of 15 µm and a Gaussian gram size distribution with no grain larger than 200 urn Thus homogenous fine microstructure shown in Fig, 8 might be expected to have a very high norrmalized ribbon quantity cast. But me casting wheel exhibited an extremely poor normalized ribbon quantity cast value of 0.3, which is much less than that of die avenge standard wheel, which has a significantly larger grain size.
The wheels described in Examole 1,2 and 3 all exhibit an average grain size less than 30u.m. but have very different microstructures. Only the wheel of Example 1 produced in accordance with the present invention and having a microstructure characterized by fine, equiaxed, recrystallized grains with a tight Gaussian gran size distribution has superior casting performance.
Casting wheels were formed by the direct hot extrusion of a tube An ingot of the; copper beryllium alloy 25 was upset hot forged to fit within the extrusion container It was then pierced, while still hot, to the internal diameter of the tube to be extruded After piercing, the billet was cooled, inspected and then reheated to the extrusion temperature of 650 oC. The size of the extrusion container was chosen to give a reduction ratio of around 10:1, to ensure that a uniformly high deformation was given to the ingot. The extruded tube was given a standard solution heat treatment and aging. It was then sliced; each slice was machined to the exact dimensions and tolerances of the casting wheel.
The resultant microstructure was found to be equiaxed and was characterized by a tight Gaussian grain size distribution, as shown in Fig. 9 The grains were recrystallized and, as such, were effectively free of dislocations associated with bom cold and hot working of these alloys
An ingot of die copper beryllium alloy 25 was hot upset forged, pierced and then hot forward extruded to a tube using the procedure described in Example 4. This tube was then cold flow formed to the required dimensions for a casting wheel, achieving a 50% reduction As Fig 4 shows a cold reduction of 20 to 70% could be used to achieve me optimum grain size The flow formed tube was given a standard solunon heat treatment, aged and machined to me required tolerances The microstructure consisted of equiaxed grams with a light Gaussian grain size distribution and an average grain size of approximately 30µm
Other mechanical working processes can be used instead of flow forming One is cold saddle forging, which has been found to result in recrystallizec. grains with an extremely tight Gaussian grain size distribution with an average grain size of 20 µm This wheel had In addition to the mechanical deformation processes described above. various heat treatment steps, carried out either between or during the mechanical deformation processes, may be utilized to facilitate processing and/or to recrystallize the quench surface grains, and to produce the hardening phases in the quench surface alloy
Having thus described the invention in rather full detail, it will be understood that such detail need not be smelly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all failing within the scope of the present invention as defined by the subjoined claims
1. A quench surface for rapid solidification of molten alloy into strip
having a microcrystalline or amorphous structure, said quench
surface having composed of a thermally conducting alloy having a
microstructure consisting of fine, equiaxed, recrystallized grains, the
average size of said grains being less than 200 um and none of said
grains being larger than 500 um, said grains having a iight Gaussian
grain size distribution.
2. A quench surface as claimed in claim 1, wherein said thermally
conducting alloy is copper-based.
3. A quench surface as claimed in claim 2, wherein said thermally
conducting alloy is a precipitation-hardened copper alloy.
4. A quench surface as claimed in claim 2, wherein said thermally
conducting alloy is dispersion-hardened copper alloy.
5. A quench surface as claimed in claim 2, wherein said thermally
conducting alloy is beryllium copper alloy.
6. A quench surface as claimed in claim 1, said alloy having a
substantially homogenous microstructure wherein said gains have
an average grain size less than 100 µm.
7. A quench surface as claimed in claim 1, said alloy having a
substantially homogenous micro structure wherein said grains have
an average grain size less than 30 µm.
8. A mechanical forming/heat treating process for making the quench
surface as claimed in claim 1, wherein said quench surface is
subjected to extrusion and then ring rolling prior to the final solution
heat treatment step.
9. A process as claimed in claim 8, wherein said quench surface is
extruded at a low temperature and medium extrusion ratio prior to
said final solution heat treatment and aging step.
10. A process as claimed in claim 8, wherein said quench surface is
subjected to hot forging and then cold forging prior to the final
solution heat treatment and aging step.
11. A quench surface substantially as herein described with reference to
and as illustrated in the accompanying drawings.
12. A mechanical forming/heat treating process for making the quench
surface substantially as herein described with reference to and as
illustrated in the accompanying drawings.
|Indian Patent Application Number||2349/DEL/1997|
|PG Journal Number||35/2010|
|Date of Filing||20-Aug-1997|
|Name of Patentee||METGLAS, INC.|
|Applicant Address||440 ALLIED DRIVE, CONWAY, SOUTH CAROLINA 29526, UNITED STATES OF AMERICA.|
|PCT International Classification Number||B22D 11/06|
|PCT International Application Number||N/A|
|PCT International Filing date|