Method for producing copper alloy ingot

A method for producing a copper alloy ingot with suppressed casting defects, segregation of components and lower oxide content comprising heating a copper alloy material in a graphite crucible to melt the copper alloy material. The molten alloy in the graphite crucible is cooled from the bottom of the crucible so that the molten alloy solidifies in a single direction.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a useful method for producing a high
 quality and sound copper alloy ingot while suppressing casting defects,
 segregation and oxide content.
 2. Description of the Prior Art
 A vacuum melting casting method has hitherto been employed for producing a
 copper alloy ingot, which is free from oxides and contains a reduced
 number of pinholes caused by dissolved oxygen or dissolved hydrogen. In
 addition, various attempts have been made, which include directional
 solidification under vacuum or in an argon gas atmosphere, in order to
 produce a sound ingot having less shrinkage cavities, segregation of
 components and other defects. The shrinkage cavities, which are usually
 caused during solidifying the molten metal, include macroscopic shrinkage
 cavities (e.g., center shrinkage cavity and final shrinkage cavity) and
 microscopic shrinkage cavities (e.g., shrinkage cavity observed in grain
 boundaries under a microscope). The conventional melting casting
 techniques commonly employ such procedure as, after the copper alloy
 material is molten in a crucible, a molten metal is poured into another
 container (casting mold) and then cooled to thereby solidify the molten
 metal.
 However, the above conventional methods for melting and casting a copper
 alloy have a variety of problems, especially because there is provided the
 step of pouring the molten metal into the casting mold. The problems
 includes poor workability and productivity, complicated operations
 required to control the molten metal temperature during the pouring step,
 the low cooling efficiency of the casting mold leading to greater feeding
 and higher equipment investment, thus resulting in higher producing cost.
 When the material is poured or molten in air rather than under vacuum or
 in an argon gas atmosphere to prevent these problems, there arises a
 problem that oxides are entrained in the step of pouring or melting the
 molten metal.
 SUMMARY OF THE INVENTION
 To solve the above problems, an object of the present invention is to
 provide a method capable of producing a sound copper alloy ingot with a
 reduced number of casting defects, segregation and oxide content at low
 cost, while improving the workability and productivity with reasonable
 molten metal control operation and minimum feeding.
 According to an aspect of the present invention, a method for producing a
 copper alloy ingot includes steps of heating a copper alloy material in a
 graphite crucible to melt the copper alloy material, and cooling the
 molten metal in the crucible from a bottom of the crucible so that the
 molten metal solidifies in a single direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present inventors have studied intensively to attain the above object.
 As a result, the inventors have found that the casting problem such as
 entrainment of oxides and gas inclusion can be prevented and internal
 quality can be improved by melting a copper alloy in a graphite crucible
 and then rapidly cooling a molten metal in the crucible, to solidify the
 molten metal in a single direction. Main features of the above method
 include using the same crucible for melting and cooling, and solidifying
 in a single direction. That is, in this method, it is not necessary to
 transfer the molten metal from the crucible to a casting mold to be
 solidified.
 Although the mechanism that the above method solves the casting problems
 described above cannot be completely explained, it can be understood that
 solidification of a molten metal in a crucible without transferring
 therefrom makes it possible to maintain the reducing zone, thereby
 preventing the molten metal from entraining the air as well as the oxide
 of the molten metal that comes into existence due to reaction of the
 molten metal with the air.
 In addition, the present invention may preferably include (1) shielding the
 copper alloy material in the crucible with inert gas such as argon gas
 until the material starts melting, and then, (2) after the material starts
 melting, covering the molten metal surface with a carbon-based substance
 such as carbon chips, carbon powders or carbon-based flux. This makes it
 possible to insulate the molten metal from the air containing moisture, to
 thereby achieve a reducing zone around the molten metal. The reducing zone
 can prevent production of oxides in the metal and absorption of hydrogen
 by the metal. Furthermore, by the above covering (2), the reducing zone to
 insulate the molten metal from the air can be attained without closing the
 crucible with a lid. Thus, if necessary, a desired additional component
 can be mixed with the molten metal in the melting step while maintaining
 the reducing zone.
 When the molten metal surface is covered with the carbon chips, carbon
 powders or the carbon-based flux, a part of such material is likely to
 deposit on the inner surface of the crucible. This is prevented by
 applying argon gas bubbling after melting, to thereby separate the carbon
 chips, carbon powders or the carbon-based flux through floatation while
 degassing from the molten metal.
 Examples of the copper alloy obtained by the present invention include
 superconductive copper alloy billets containing about 13 to 15.8% of Sn
 and Cu as balance. In order to improve the superconductivity of the copper
 alloy, the copper alloy may further include a small amount (e.g., 0.3% or
 less) of an element such as Ti. Ti operates as the deoxidizing agent
 removing the residual oxygen in the molten metal. Therefore, when the
 element is added before conducting the argon gas bubbling in the method of
 the present invention, the element is likely to react with a trace amount
 of oxygen or moisture contained in the argon gas and is oxidized. The
 oxided element floats to the molten metal surface during the bubbling,
 thus canceling the effect of adding the element. In order to avoid such a
 disadvantage, it is preferred, when a reducing element, i.e., an element
 having a strong affinity with oxygen, such as Ti is added as an alloy
 component, to add it after the completion of the argon gas bubbling.
 In the method of the present invention, to rapid cool from the bottom of
 the molten metal in the crucible for directional solidification, it is
 preferred to spray cooling water (hereinafter also referred to as
 "showering") to the outer surface of the crucible while moving the
 crucible relative to the cooling water spray in a single direction. This
 unidirectional solidification of the molten copper alloy can prevent the
 occurrence of macroscopic shrinkage cavities. In addition, efficient
 cooling of the molten metal through the graphite crucible having a high
 heat transmission can minimize the occurrence of microscopic shrinkage
 cavities, pinholes, segregation of components and other defects. Also, the
 solidification method can provide fine and desirable microstructure to the
 final alloy ingot. Specifically, this method includes lowering the
 graphite crucible containing the molten alloy into cooling water sprayed
 from a showering device, or raising a showering device along the outer
 surface of the fixed crucible.
 According to the method of the present invention, it is also effective to
 apply electromagnetic stirring to a portion of the molten metal in the
 crucible above the interface of solidification which is being showered.
 This activates the molten metal in this region so as to further suppress
 formation of microscopic shrinkage cavities and segregation, and further
 refine the structure of in the final alloy ingot.
 The copper alloy used in the present invention is not specifically limited
 and there can be used various bronze alloys for general purpose, in
 addition to the above-described Cu--13-15.8%Sn alloy (a small amount of Ti
 is added as required).
 With the conventional casting method, the Cu--Sn copper alloy described
 above has been limited to such constitutions as the Sn content is not
 higher than about 14% by weight due to the problems described previously
 (e.g., the occurrence of segregation), although solid solubility limit of
 Sn into a solid solution is about 15.8% by weight as shown in FIG. 1
 (Cu--Sn equilibrium state diagram). The method of the present invention,
 however, has such an effect as the content of the alloy element can be
 increased to 15.5% by weight that is near the solid solubility limit,
 since the disadvantages such as segregation can be avoided. The increased
 amount of the additional Sn improves the superconducting performance of
 the alloy.
 Next, the present invention will now be described in detail below by way of
 a preferred embodiment. The following embodiment is not intended to limit
 the scope of the present invention, and it will be understood that any
 modification or alteration of the features herein is included in the scope
 of the present invention. The method of the present invention may also be
 referred to as a "Mizuta system", hereinafter.
 FIG. 2 schematically shows a construction of a casting facility for
 carrying out the Mizuta system. The facility has a graphite crucible 1, a
 high frequency coil 2, a heat insulating sleeve 3, and a showering device
 4. The showering device 4 is provided with associated devices such as a
 water tank 5, pumps P1, P2, motor-operated valves M1, M2, flow control
 valves V1 to V18 and spay nozzles 8 arranged in a circle (15 nozzles in
 this drawing). In the showering device 4, the pumps P1, P2 pump up cooling
 water from the water tank 5, the flow rate of the cooling water is
 controlled by the motor-operated valves M1, M2 and the flow control valves
 V1 to V18, and the cooling water is sprayed from the spray nozzles 8.
 Although not shown in detail in FIG. 2, the graphite crucible 1 is provided
 with a hydraulic cylinder with a support base made of heat-resistant
 cement being secured at an end of a rod that extends from the hydraulic
 cylinder. The graphite crucible 1 is placed on the support base so that
 the graphite crucible 1 can be raised and lowered by extending and
 retracting the hydraulic cylinder. The hydraulic cylinder has a stroke of
 motion indicated with L in the drawing. By the cylinder movement, the
 graphite crucible 1 can move up until the bottom of the graphite crucible
 1 reaches the top of the high frequency coil (the position is shown in
 FIG. 2 as "A") and down until the top of the graphite crucible 1 reaches a
 little below the top of the showering device 4 (the position is shown in
 FIG. 2 as "D").
 Procedure of the Mizuta system using the casting facility described above
 will be described below. First, the graphite crucible 1 is raised to a
 proper height in such a manner that the position of the upper end of the
 crucible is set at the position indicated by "A", and specified masses of
 pure copper and pure tin (to obtain the proportion of pure tin of, for
 example, from 13 to 15.5% by weight) are charged into the crucible. Then
 the crucible is lowered into the high frequency coil 2 to start melting by
 induction heating. The position of the upper end of the crucible at the
 start of melting is indicated by "B" in FIG. 2.
 After charging pure copper and pure tin into the graphite crucible 1, a lid
 (not shown) is put on the graphite crucible 1 and the inside and
 surrounding of the crucible shielded with argon gas. When the pure copper
 begins to melt, the lid is removed and carbon powders (or carbon-based
 flux) are sprinkled on the material surface. Thereafter, the material
 (i.e., molten metal) is prevented from the absorption of oxygen in the air
 by carbon chips or carbon powders, to let the whole material melt while
 occasionally supplying carbon chips or carbon powders.
 When the copper alloy material is completely melted, the temperature is
 raised to the temperature 100.degree. C. higher than the solidification
 starting temperature, while monitoring the molten metal temperature. This
 makes it possible that the feeding operates effeciently. In addition,
 since the temperature is not too high, an excessive amount of hydrogen
 will not be absorbed by the molten metal due to overheat. Then argon gas
 is supplied through a graphite pipe at a proper flow rate to carry out
 bubbling for several minutes hereby to cause the carbon chips or carbon
 Powders deposited on the inner surface of the crucible to float
 sufficiently. After the bubbling, an alloy element (the third element)
 such as Ti is charged into the molten metal as required, and the crucible
 is set stationary so that the third element diffuses into the molten
 metal.
 Then while lowering the graphite crucible 1 at a proper speed, cooling
 water is sprayed from the nozzles of the showering device 4 to thereby
 rapidly cool down the external surface of the crucible. (In FIG. 2, the
 position of the upper end of the crucible at the start of cooling is
 indicated by "C".) At this time, the following three zones are formed in
 the copper alloy contained in the crucible: a completely solidification
 zone formed from the bottom of the crucible; a melting zone; and a
 transition zone formed between the completely solidification zone and the
 melting zone. The alloy in the transition zone has a temperature ranging
 from the solidification starting temperature (at the upper surface of the
 zone) to the solidification completion temperature (at the lower surface
 of the zone). The molten metal in the melting and transition zones may be
 stirred. Preferably, the molten metal being solidified in the transition
 zone is moved by stirring, for example, with the use of an electromagnetic
 stirrer by supplying high-frequency power. The stirring of the molten
 metal in this zone can further suppress formation of microscopic shrinkage
 cavities and inverse segregation of tin, and makes the structure finer.
 Cooling is stopped when the entire copper alloy in the crucible is
 completely solidified. (In FIG. 2, the position of the upper end of the
 crucible at the end of cooling is indicated by "D".) Although FIG. 2 shows
 the construction where the graphite crucible containing the molten copper
 alloy is lowered into cooling water sprayed from the showering device, the
 facility is not limited to the construction shown in FIG. 2. Such a
 construction may be used as the showering device is raised along the outer
 surface of the fixed crucible. To sum up, applied can be any construction
 where the crucible moves relative to the cooling water spray in a single
 direction.
 Examples of solidification structure of a copper alloy ingot cast by the
 casting equipment described above are shown in FIGS. 3 to 5. These
 photographs show microstructures (magnified 100 times) at height of 600 mm
 from the bottom of a copper alloy ingot (Cu, 14% Sn, 0.3% Ti: 180 mm
 (diameter), 700 mm high) obtained without electromagnetic stirring. FIG. 3
 shows a sample taken from a position of 72 mm in a radial direction, FIG.
 4 shows a sample taken from a position of 36 mm in a radial direction, and
 FIG. 5 shows a sample taken from the center. These photographs show
 portions where shrinkage cavities were observed, and only slight shrinkage
 cavities can be seen as a whole. Although not shown in the photographs,
 similar structure was seen at a height of 150 mm. Therefore, it is
 supposed that the ingot has a similar structure at any height except for
 the portion near the bottom.
 The present inventors studied about the influence of the electromagnetic
 stirring on the inverse segregation when a copper alloy ingot measuring
 180 mm in diameter and 700 mm in height is cast by using the casting
 facility described above. The results are shown in FIG. 6. In the drawing,
 the reference symbol ".circle-solid." shows a plot of measurement when
 electromagnetic stirring was applied, and the reference symbol ".times."
 shows a plot of measurement when electromagnetic stirring was not applied.
 FIG. GA shows measurements at a height of 150 mm above the bottom of the
 ingot, FIG. 6B shows measurements at a height of 300 mm above the bottom
 of the ingot, FIG. 6C shows measurements at a height of 450 mm above the
 bottom of the ingot, and FIG. 6D shows measurements at a height of 600 mm
 above the bottom of the ingot. Measurements at sampling positions 1 to 5
 in the radial direction shown in FIGS. 6A to 6D are taken at 72 mm from
 the center for 1, at 57 mm from the center for 2, 36 mm from the center
 for 3, 18 mm from the center for 4 and 0 mm (i.e., center) for 5.
 As is apparent from these results, it is effective in restricting
 segregation to solidifying the molten metal while applying electromagnetic
 stirring.
 Examples of solidification structure of a copper alloy ingot
 (Cu--14%Sn--0.3%Ti: 180 mm (diameter), 700 mm high) obtained while being
 electromagnetically stirred are shown in FIGS. 7 to 9 (microscope
 photographs). These photographs show microstructures (magnified 100 times)
 at height of 600 mm from the bottom. FIG. 7 shows a sample taken from a
 position of 72 mm in a radial direction, FIG. 8 shows a sample taken from
 a position of 36 mm in a radial direction, and FIG. 9 shows a sample taken
 from the center. These photographs show that the number of shrinkage
 cavities are less than those shown in FIGS. 3 to 5 and also the cast
 structure is finer.
 FIGS. 10 to 13 are photographs showing microstructures of a copper alloy
 ingot (Cu--14%Sn--0.3%Ti) containing a titanium oxide entrained during
 pouring operation in a conventional method. FIG. 10 shows a secondary
 electron image, FIGS. 11 to 13 respectively show a C--K .alpha. image, a
 Ti--K .alpha. image and an O--K .alpha. image by X-ray microanalyzer.
 FIGS. 14 to 16 are photographs showing macroscopic shrinkage cavities
 generated in a copper alloy ingot (Cu--14%Sn--0.3%Ti) produced by the
 conventional metal mold casting method. FIG. 14 shows a sample taken from
 a height of 200 mm above the bottom, FIG. 15 shows a sample taken from a
 height of 340 mm above the bottom of the ingot, and FIG. 16 shows a sample
 taken from a height of 470 mm.
 These results show that the Mizuta system of the present invention produces
 better microstructure than the conventional method that produces copper
 alloy ingots having such defects as shown in FIGS. 10 to 16.
 FIGS. 17 to 19 are graphs showing a concentration distributions of tin in a
 radial direction and in a vertical direction, measured at sections at
 height of 0 mm, 150 mm, 300 mm, 450 mm and 600 mm of copper alloy ingots
 measuring 180 mm in diameter and 700 mm in height produced by the
 conventional method (vacuum melting casting method). FIGS. 17 to 19
 respectively show the ingots containing 13%, 14% and 15% Sn.
 FIGS. 20 and 21 are graphs showing a concentration distributions of tin in
 a radial direction and in a vertical direction, measured at sections at
 height of 0 mm, 150 mm, 300 mm, 450 mm and 600 mm of copper alloy ingots
 (all containing 15%Sn) measuring 180 mm diameter and 700 mm in height
 produced by the Mizuta system.
 As described above, the method according to the present invention is
 capable of producing a sound copper alloy ingot with suppressed casting
 defects, segregation and oxide content at low cost, while improving the
 workability and productivity with reasonable molten metal control
 operation and minimum feeding.
 As will be apparent from FIGS. 17 to 21, the Mizuta system of the present
 invention also suppresses segregation of tin as compared with the
 conventional method.
 The present invention has been described with reference to the present
 embodiments. Obviously, modifications and alterations will occur to others
 upon reading and understanding the proceeding detailed description. It is
 indeed that the present invention be construed as including all such
 modifications and alterations insofar as they come within the scope of the
 attended claims or the equivalents thereof.