Abstract:
A method and apparatus for casting wherein molten metal is treated to create conditions of whole volume solidification in the mold. The treating of the molten metal can include the application of vibration in specified manner under specific frequency and amplitude for causing multiple sites of crystallization throughout the mold, or the treating can be by use of activators which are formed during casting, either directly from the molten metal stream by dispersion or from a mineral which concurrently with casting is milled, to a finely ground powder, dehydrated and fed into the molten metal throughout the metal to speed solidification and crystallization at multiple sites. Solidification speed can be more than doubled while eliminating pouring defects and improving the cast metal structure. The methods are applicable to various methods of casting including continuous casting and the methods of vibrating and use of activators can be combined with improved results.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. provisional application No. 60/935,152 filed Jul. 27, 2007 and U.S. provisional application No. 61/004,115 filed Apr. 11, 2008. The disclosures of these provisional applications are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention involves new methods and apparatus directed to improving the quality of both casts and ingots of ferrous and non-ferrous metals involving treating the molten metal to create conditions of whole volume solidification (crystallization) in the mold through the use of either mechanical vibrations, including direct agitation of the liquid steel, or through the use of suspension pouring (activators). 
       BACKGROUND AND SUMMARY 
       [0003]    V. A. Povidaylo et al. report in their 1963 article “Casting in Vibration Molds” that the idea of improving the properties of cast steel by vibration in the process of solidication was mentioned for the first time by D. K. Chernov back in 1868. Reportedly, practical vibration of iron chill molds was realized by K. M. Korol&#39;kov and N. I. Smirnov under industrial conditions when casting pistons of aluminum alloys in 1938. With the use of vibration there was a reduction in waste due to gas blow holes and also an improvement in the quality of castings, explained by the overcoming of surface tension of the molten metal and as a result of facilitating the flow of the melt and filling the forms. Metal castings cast in a vibrating form showed that the structure is a much finer structure and has a greater density as compared with the structure of an alloy obtained by casting under conventional conditions. There have been numerous proposals for exploiting vibration in casting. For example, the present inventors earlier U.S. Pat. No. 5,799,722 in the field of continuous casting involved the application of vibration to not only improve the quality of the continuous cast but also increase the productivity of the apparatus as compared with known methods and apparatus. However, in the known methods of casting with vibration, both casts and ingots, there remains considerable losses with physical and chemical heterogeneity in the cast metal structure. 
         [0004]    The use of suspension pouring (activators) to speed solidification of cast metal is known. Made from steel activators presently used industrially in casting are produced in specialized plants, transported to the casting production site and have a tendency to oxidize, interact with the molten steel being cast and thus cause non-metallic inclusions in the steel end product worsening the steel ingot quality, increasing its cost. 
         [0005]    There is a need for improved casting methods and apparatus, directed to improving the quality of both casts and ingots, which avoid the aforementioned drawbacks and particularly which reduce losses, increase productivity, eliminate pouring defects and improve the product quality. The present invention addresses this need. According to the disclosed, preferred embodiment of the invention, the improved method of casting of the invention comprises pouring molten metal into a mold and treating the molten metal to create conditions of whole volume solidification (crystallization) in the mold. The treating includes at least one of: 
         [0006]    vibrating the molten metal in the mold using a vibroplate supporting the mold, the vibroplate being vibrated at a frequency of 50 to 60 Hertz and an amplitude of 0.1 to 0.2 mm; 
         [0007]    vibrating the molten metal in the mold by direct agitation of the molten metal using a vibrating waveguide in contact with the molten metal, the waveguide being vibrated at a frequency of 50 to 60 Hertz and an amplitude of 1.0 to 2.0 mm; 
         [0008]    adding activators to the molten metal so they are dispersed throughout the mold and act as solidification sites to speed solidification. 
         [0009]    Each of these treatments has been found to create conditions of whole volume solidification (crystallization). The vibrations at the frequencies and amplitudes indicated keep stirring the molten metal thus creating the conditions of whole volume solidification. In addition, the invention includes the combined use of vibration and suspension pouring (activators) in accordance with the invention for better cast and ingot quality. 
         [0010]    The treating involving vibrating the molten metal in an example embodiment using the vibroplate is particularly useful in casting steel and cast iron ingots of ordinary weight, e.g., up to approximately 10 tons. The direction of the vibration is in a longitudinal axial direction of the solidifying ingot. 
         [0011]    According to another embodiment, it has been found that for larger, multi-ton ingots, direct agitation of the molten metal is more advantageous in creating the conditions of whole volume solidification (crystallization). As compared with using a vibroplate method, this method allows the use of lower powered vibrators. In the example embodiment the waveguide can be made of either a refractory ceramic material, a steel rod, or a water cooled agitator. 
         [0012]    The application of vibration for casts and ingots using both methods of vibroplate and waveguide is commenced only after an interval of 5 to 30 minutes, after filling up the mold, the time interval depending on the mold size. The application of vibration is then commenced for a period of 5 to 30 minutes then stopped/paused for a period of 5 to 30 minutes. This cycle of vibration and pausing/stopping is continued until the end of solidification. This method of the application of vibration with the delay in commencing vibration and the pauses in vibration is necessary to allow the growth of dendrites in the melt during the pauses which are creating the new sites of crystallization throughout the mold and then breaking them up during the application of the vibration. The time intervals of delay in starting the vibration after filling the mold and the pattern at which the pauses of the vibration are repeated depends on the size of the mold, e.g., in greater size molds a greater time interval would be required. 
         [0013]    When the treating of the molten material involves adding activators, the activators are formed during casting in accordance with the invention. In one embodiment the forming includes dispersing droplets of molten metal from a molten metal stream being poured into the mold to form activators. The dispersing includes impinging a stream consisting of inert gas or water onto the molten metal stream to form the droplets and cooling the droplets to form activators. In another embodiment, forming the activators during casting involves milling a material to a finely ground powder, dehydrating the powder and feeding the powder into the molten metal. The activator is stirred in or blown in so that it is dispersed throughout the mold and acts as solidification sites. This material for the activator is preferably a low cost natural mineral with high sublimation value. In the example embodiment sodium chloride is utilized as the material for the activators. 
         [0014]    These activators do not disintegrate during casting and do not chemically interact with the molten metal. The activators are vaporized by the molten metal. According to the method of the invention, the vapor of the vaporized activators is recovered during casting, condensed, dehydrated and then reused many times to speed solidification of molten metal and create conditions of whole volume solidification. 
         [0015]    In one form of the invention, the apparatus of the invention for casting comprises a mold into which molten metal can be poured, and a device which forms activators during casting and provides the activators to the molten metal for removing superheat from the molten metal to speed solidification. In another form of the apparatus, a waveguide is provided for contacting the molten metal in the mold for transmitting vibration directly into the molten metal at a frequency of 50 to 60 Hertz and an amplitude of 1.0 to 2.0 mm, or a vibroplate is provided for vibrating the mold at a frequency of 50 to 60 Hertz and an amplitude of 0.1 to 0.2 mm, in the longitudinal axial direction of the casting. 
         [0016]    These and other features and advantages of the present invention will become more apparent from the following detailed description of embodiments of the invention taken with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a side view of a first apparatus of the invention including an ingot mold for casting, which may be used in practicing the method of the invention, the mold being supported on an assembly of a vibroplate and a vibrator for vibrating liquid metal solidifying in the mold with the requisite frequency and amplitude for achieving whole volume solidification (crystallization) of the casting. 
           [0018]      FIG. 2  is a side view, partially in cross section, showing another apparatus of the invention comprising an ingot mold supported on a stationary support with an assembly of a vibrator and a waveguide (agitator) connected thereto and extending into liquid metal in the mold from the top of the mold for direct agitation of the solidifying melt at the requisite frequency and amplitude for creating whole volume solidification. 
           [0019]      FIG. 3  is a side view, partially in cross section, of a further casting apparatus of the invention having a suspension pouring (activators) arrangement for forming activators during casting. 
           [0020]      FIG. 4  is a schematic illustration of another casting apparatus of the invention for forming activators during casting from a low cost natural mineral by milling the mineral to a finely ground powder, dehydrating the powder to form activators and feeding the activators into the molten metal, and for collecting the vapor of vaporized activators from the mold, and condensing and reusing it multiple times in treating the molten metal. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring now to the drawings, the disclosed, preferred embodiments of the method and apparatus of the invention involve the application of either vibration or suspension pouring (activators) in a specific, optimal manner which results in a whole volume solidification (crystallization) contributing to a considerable reduction in losses, increase in speed of solidification and elimination of pouring defects, thus achieving considerable improvements of the product quality and a reduction in cost.  FIGS. 1 and 2  depict apparatus for application of vibration in the method of the invention. 
         [0022]    The vibration is applied in the methods with a frequency of 50 to 60 Hertz to the solidifying melt, such as steel or cast iron ingot, in an axial direction, A-A, until full solidification. For steel and cast iron ingots of ordinary weight, the vibration from vibrator  4  is preferably applied via a vibroplate  1  on which the mold  2  is supported as shown in  FIG. 1 . The vibrator  4  in  FIG. 1  vibrates the vibroplate at an amplitude of 0.1 to 0.2 mm at the aforementioned frequency of 50 to 60 Hertz in order to create conditions of whole volume solidification. For multi-ton ingots of steel and cast iron, it has been found advantageous to apply the vibration using a waveguide (agitator)  3 ,  FIG. 2 , in contact with the liquid metal for direct agitation of the liquid metal. This enables the use of lower power vibrators as compared with using a vibroplate arrangement as in  FIG. 1 . The waveguide can be made of a refractory ceramic material, a steel rod, or a water cooled agitator. The waveguide size is based on the size of the ingot. The cross sectional area of the waveguide is preferably ⅕ to ⅓ of the cross sectional area of the mold. The required amplitude of vibration of the waveguide  3  is 1.0 to 2.0 mm at a frequency of 50 to 60 Hertz. The vibrators  4  can be any known vibrator, e.g., pneumatic, electromagnetic, mechanical, hydraulic, producing the desired frequency of 50 to 60 Hertz and allowing vibration amplitude control to provide the required amplitude of vibration in order to create conditions of whole volume solidification of the casting. 
         [0023]    The application of vibration for casts and ingots using both methods of vibroplate and waveguide is commenced only after an interval, e.g. 5-30 minutes, after filling up the mold. The application of vibration is applied for a period of 5 to 30 minutes and then stopped for a similar period of time. This cycle of vibrating and pausing vibration continues until the end of solidification. This method of the application of vibration with the pauses is necessary to allow the growth of dendrites in the melt during the pauses which are creating the new sites of crystallization throughout the mold and then breaking them up during the application of the vibration. The time intervals at which the vibration starts after filling and the pattern or intervals at which it repeats depends on the size of the mold, e.g., in greater size molds a greater time interval would be required. 
         [0024]    Where treating to create conditions of whole volume solidification involves the use of suspension pouring (activators), the activators used in the method are formed during casting thereby avoiding a tendency to oxidize which can lead to non-metallic inclusions in the steel end product and worsening of the steel ingot quality. In one form of the invention depicted in  FIG. 3 , the activators  5  are formed during casting by dispersing droplets of molten metal from a molten metal stream  6  being poured from a ladle or other container  7  into the mold  8  through a disperser  9 . The disperser directs a pressurized flow of either inert gas (argon or nitrogen) or water onto the molten metal stream being poured through the disperser to form the droplets and cools the droplets as they contact the water-cooled wall  10  of the disperser, to form the activators. The activators are deflected from the wall back into the molten metal stream thus removing superheat from the liquid metal and forming crystallization sites and thus creating a condition of whole volume solidification of the molten metal in the mold. The inert gas or water in the example embodiment is impinged on the molten stream in an amount in order to cause a partial, e.g., ⅕ to ⅓ dispersion of the liquid metal stream into activators. 
         [0025]    In the form of the invention shown in  FIG. 4 , the activators are formed concurrently with casting and introduced into the pouring mold  11  using a low cost natural mineral/activator with high sublimation value, e.g. a finely ground salt powder, sodium chloride, is used in the example embodiment, which during the process is undergoing dehydration and then is introduced into the molten metal by means of a vibratory feeder  12 . Bulk sodium chloride supplied to a bunker  13  is milled in mill  14  to a finely ground powder and the powder dehydrated in dryer  15  before being fed by the vibratory feeder into the molten metal in mold  11  via apertures in a steam catcher  16  shown in  FIG. 4 . The quantity of activator used is approximately 100 to 150 kilograms per ton of cast metal. The activator decreases the superheat temperature of the molten metal and acts as sites for crystallization creating conditions for whole volume solidification (crystallization) in the mold. The activator does not disintegrate and does not chemically interact with liquid metal particles. The natural mineral&#39;s steam or vapor is captured by the steam catcher  16 , conduit  17  and suction pump  18 . The recovered vapor is condensed in condenser  19  from where it is recirculated through the mill  14  and dryer  15  and feeder  12  for reuse. 
         [0026]    These methods of forming the activators during casting are shown in top pouring of ingots but could be applied in the case of bottom pouring and even for continuous casting. In case of bottom pouring each mold is supplied with individual (or shared) steam catchers. The methods are environmentally safe and could be used with all types of continuous casting machines and in ingot casting. By producing the activators during the casting process according to the invention, the tendency of the activators to oxidize is avoided and transportation needs and cost are lowered. Further, it is has been found that with this invention defects in steel ingots can be eliminated and the increased speed of solidification allows for the combination of rolling and continuous casting processes. 
         [0027]    The application of vibration to the solidifying metal according to the invention causes several physical processes common to wave energy, e.g., cavitation and pressure changes, which allow the breaking down of growing crystallites, and it increases the fluidity of the melt, which is of great importance in production of complex configuration casts. The resulting whole volume solidification contributes to considerable decrease in casting defects. 
         [0028]    In steel ingots produced by the method of the invention there is improvement of the structure, considerable decrease in grain size, improvement in physical and chemical heterogeneity-reduction of carbon and other chemical element segregation as proved by industrial trials of 6 ton ingots, elimination of gas and of non-metallic inclusions and decrease in the depth of the shrinkage pipe, and an improvement in crystallic heterogeneity. The density and the ability of steel to withstand corrosion are also considerably increased. 
         [0029]    In cast iron ingots produced with the method of the invention graphite inclusions were considerably lessened and were distributed more evenly. There was a considerable decrease of gas entrapment and of non-metallic inclusions, an improvement of grain structure (decrease in grain size), and an overall improvement of mechanical qualities and ability to withstand corrosion. 
         [0030]    As mentioned above, the application of suspension pouring (activators) in accordance with the invention permits doubling the solidification speed, increases the coefficient of mold usage, and eliminates pouring defects, e.g. chemical and physical heterogeneity. Considerable decrease in the depth of the shrinkage pipe and considerable improvement in the metal structure were also observed. When water is being used in the method it is possible to reuse the forming steam for industrial or everyday needs. Photographs appearing in the aforementioned provisional applications illustrate the improvements of microstructure of casts and ingots produced by non-continuous and continuous casting using the methods of the present invention. The application of vibration and of suspension pouring (activators) in the disclosed embodiments is with respect to the top of the mold but various other ways are possible as noted above. These include bottom of the mold, per mold, per unit, to the pouring car and continuous casting. That is, the described methods are applicable to all methods of production (either top or bottom pouring). Application of the vibration is equally effective in production of ingots of ferrous and non-ferrous metals: alloys, casts, and ingots. The suspension pouring is more useful for production of steel ingots. 
         [0031]    The described method involving using a natural mineral for the activators has been tried on an industrial scale and the results showed considerable improvements in steel quality and primary cast thickness. Photos of microstructure evidencing this improvement are found in the aforementioned priority U.S. provisional applications. The use of the natural mineral has been found cost effective as well. 
         [0032]    While we have shown and described only several embodiments in accordance with the invention, it will be understood by the skilled artisan that the present invention is not limited to the disclosed embodiments, but could be used in connection with various other arrangements. Therefore, we do not wish to be limited to the details shown and described herein but intend to encompass the methods and apparatus of the invention as described in the accompanying claims.