Patent Publication Number: US-2015064059-A1

Title: Process for producing cast-metal object, and cast-metal object

Description:
TECHNICAL FIELD 
     The present invention relates to a process for producing a cast-metal by solidifying a molten metal in a casting mold and to a cast-metal object produced by the process for producing the same. 
     BACKGROUND ART 
     When a cast-metal object is to be produced, various casting defects (shrinkage cavity, blow defect and the like) might occur in a cast-metal object. Moreover, due to coarsening of a crystal structure in a specific portion of a cast-metal object, mechanical characteristics or strength of the cast-metal object might deteriorate. In response to that, improvement of a quality of the cast-metal object has been required by suppressing occurrence of a casting defect and a coarse crystal structure. Particularly, in precision casting, a shape by casting is to be a shape of an end product, and since the casting defect and the coarse crystal structure remain as they are in the product, the quality of the cast-metal object needs to be further improved. 
     In a mold for tire molding produced by precision casting, too, casting defects (shrinkage cavity, for example) might occur at a specific portion. The mold for tire molding is a mold for molding a tire used in an automobile or the like and is cast by a casting mold made of plaster in general. In casting, the mold for tire molding is molded in a casting mold, and fine projections are formed on the mold for tire molding for forming recess portions (grooves and the like) in a tire. At that time, the projections are formed by filling a molten metal in recess portions (grooves and the like) formed in the casting mold and solidifying it. However, as will be described later, it is difficult to produce a perfect mold for tire molding with no shrinkage cavity. 
     That is, a shrinkage cavity occurs in a space (closed space) formed between a surface of the casting mold and an equal solidification time curved surface of the molten metal. The equal solidification time curved surface is a curved surface connecting positions where the molten metal is solidified in the same time in the casting mold. By making the surface of the casting mold and the equal solidification time curved surface orthogonal to each other, formation of a closed space and occurrence of a shrinkage cavity can be prevented. Thus, when a mold for tire molding is to be cast, the surface of the casting mold is generally conformed to the direction of the gravity and set so that a direction (solidification direction) in which solidification of the molten metal proceeds from top to bottom (or from bottom to top). However, since the casting mold for a mold for tire molding has a plurality of fine concave portions, a closed space might be generated between a surface of the concave portion and the equal solidification time curved surface. Thus, it is difficult to completely prevent occurrence of a shrinkage cavity. 
     On the other hand, a casting method for coping with occurrence of a casting defect by providing a hole, a slit or a metal member in a casting mold has been known (see PTL 1). 
     However, in the prior-art casting method, it is necessary to specify in advance a portion in which a closed space is formed by solidification simulation or the like. Moreover, since a hole, a slit, or a metal member is provided in the casting mold, time and labor for casting tend to increase. After casting, since it is necessary to adjust a shape of the cast-metal object by manual finishing in a portion corresponding to the hole, the slit or the metal member, time and labor after casting also increase. Therefore, coping with occurrence of the casting defect more simply is required. In addition, even if the cast-metal object is produced with the prior-art casting method, there is a concern that a casting defect occurs in accordance with a casting method. Moreover, it is difficult to improve a crystal structure of the cast-metal object, and there is a concern that a coarse crystal structure might be generated. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laid-Open No. 2005-169436 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention was made in view of the above-described prior-art problems and has an object to suppress occurrence of a casting defect in a cast-metal object and to make a crystal structure of the cast-metal object fine. 
     Solution to Problem 
     The present invention is a process for producing a cast-metal object by solidifying a molten metal in a casting mold and is a process for producing a cast-metal object including the steps of melting a metal material generating a free crystal during solidification, pouring the molten metal including a melted metal material into the casting mold, and solidifying the molten metal while depositing the free crystal in the molten metal on a surface of the casting mold. 
     Moreover, the present invention is a cast-metal object produced by the process for producing a cast-metal object of the present invention. 
     Advantageous Effects of Invention 
     According to the present invention, occurrence of a casting defect in a cast-metal object can be suppressed. Moreover, a crystal structure of the cast-metal object can be made fine. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are diagrams illustrating how solidification of a molten metal proceeds. 
         FIGS. 2A and 2B  are diagrams illustrating how solidification of the molten metal proceeds. 
         FIGS. 3A and 3B  are diagrams illustrating how solidification of the molten metal proceeds. 
         FIGS. 4A and 4B  are diagrams illustrating how solidification of the molten metal proceeds. 
         FIGS. 5A and 5B  are diagrams for explaining occurrence of a casting defect. 
         FIGS. 6A and 6B  are diagrams for explaining occurrence of the casting defect. 
         FIGS. 7A and 7B  are diagrams for explaining occurrence of the casting defect. 
         FIGS. 8A and 8B  are diagrams for explaining occurrence of the casting defect. 
         FIGS. 9A to 9C  are sectional views illustrating a process for producing a cast-metal object of an embodiment of the present invention. 
         FIGS. 10A to 100  are sectional views illustrating a cast-metal object in which a shrinkage cavity occurs. 
         FIGS. 11A to 11C  are sectional views illustrating a producing process of the cast-metal object using the gravity. 
         FIGS. 12A to 12C  are sectional views illustrating the cast-metal object in which a shrinkage cavity occurs. 
         FIG. 13  is a sectional view illustrating the cast-metal object produced by using a centrifugal force. 
         FIGS. 14A to 14C  are sectional views illustrating a producing process of the cast-metal object by low-pressure casting. 
         FIGS. 15A to 15C  are sectional views illustrating the cast-metal object in which a shrinkage cavity occurs. 
         FIGS. 16A to 16C  are views illustrating the cast-metal object produced by a casting mold. 
         FIGS. 17A to 17C  are sectional views of the casting mold used in a first production test. 
         FIGS. 18A to 18C  are sectional views of the casting mold used in a second production test. 
         FIGS. 19A to 19C  are sectional views of the casting mold used in a third production test. 
         FIGS. 20A to 20C  are sectional views of the casting mold used in a fourth production test. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of a process for producing a cast-metal object and a cast-metal object of the present invention will be explained by referring to the attached drawings. 
     In the process for producing a cast-metal object of this embodiment, a molten metal obtained by melting a metal material is poured into a casting mold, and the molten metal is solidified in the casting mold so as to produce a cast-metal object. By means of this process for production (casting method), the cast-metal object of this embodiment is produced. An example in which the cast-metal object is a mold for molding a tire (mold for tire molding) and the mold for tire molding is to be produced will be explained below. 
       FIGS. 1A to 4B  are diagrams illustrating how solidification of the molten metal proceeds. In  FIGS. 1A to 4A , a molten metal  1  during solidification in a casting mold (not shown) is schematically illustrated.  FIGS. 1B to 4B  are graphs illustrating a relationship between each position (distance (lateral axis) of a solidification direction S) and a temperature (vertical axis) in each of  FIGS. 1A to 4A . 
     As illustrated in  FIGS. 1A and 1B , with gradual lowering of the temperature of the molten metal  1 , solidification of the molten metal  1  proceeds. Since the molten metal  1  is solidified toward the solidification direction S, a solid phase  2  is gradually formed in the casting mold. 
     A boundary between the solid phase  2  and a liquid phase  3  of the molten metal  1  is a solid-liquid coexisting region  4  in which the solid phase  2  and the liquid phase  3  coexist. In the solid-liquid coexisting region  4 , a dendrite (dendrite  5 ) grows. The dendrite  5  is a crystal growing from the solid phase  2  toward the liquid phase  3  like a tree branch. In the periphery of the dendrite  5 , concentration of an element added to the molten metal  1  might locally rises, and a melting point of the molten metal  1  might locally lower. Moreover, an external force (micro vibration or the like) might act on the dendrite  5 . As a result of them, a tip end portion of the dendrite  5  is chipped, and a fraction-state crystal (free crystal  6 ) is formed. The free crystal  6  is made of a fine crystal freed from the solid phase  2  (here, the dendrite  5 ) and floats in the molten metal  1 . 
     As described above, while solidification of the molten metal  1  proceeds, the free crystal  6  is generated in the boundary of the solid phase  2  and the liquid phase  3  (solid-liquid coexisting region  4 ) and is supplied into the molten metal  1 . Moreover, the free crystal  6  is pushed out (or, expelled) in the solidification direction S along with the growth of the dendrite  5  and is moved in the solidification direction S in the molten metal  1 . In the process for producing a cast-metal object of this embodiment, a quality of the cast-metal object is improved by making use of the free crystal  6  generated in the molten metal  1  when the molten metal  1  is solidified. 
     Here, the metal material used in casting is usually an alloy in which a plurality of types of elements is blended and one or more additive elements are mixed with a major element. By preparing an alloy, a melting point of the metal material lowers, and a casting performance of the molten metal  1  is adjusted. Moreover, a mechanical property and strength of the cast-metal object are adjusted. When the molten metal  1  including a metal material (alloy) is solidified, a difference occurs between a temperature (TS) at start of solidification and a temperature (TF) when solidification is completed. This temperature difference is called a solidification temperature range K and it changes depending on the metal material. 
     The wider the solidification temperature range K becomes (see  FIGS. 1B and 2B ), the wider the solid-liquid coexisting region  4  becomes and the longer the dendrite  5  grows. On the other hand, the narrower the solidification temperature range K becomes (see  FIGS. 3B and 4B ), the narrower the solid-liquid coexisting region  4  becomes and the shorter the dendrite  5  becomes. The longer the dendrite  5  grows, the tip end portion can become easily chipped and thus, more free crystals  6  are generated. However, in the molten metal  1  including the metal material in which the free crystal  6  is likely to be generated (see  FIGS. 1A and 3A ), the free crystal  6  is frequently generated. In the molten metal  1  including the metal material in which the free crystal  6  is unlikely to be generated (see  FIGS. 2A and 4A ), generation of the free crystal  6  is suppressed. Thus, a generation amount of the free crystal  6  changes in correspondence with the solidification temperature range K and the metal material. 
     In the molten metal  1  in which the solidification temperature range K is wide and the free crystal  6  is likely to be generated (see  FIGS. 1A and 1B ), the generation amount of the free crystal  6  becomes larger than the generation amounts of the free crystals  6  of the other molten metals  1  (see  FIGS. 2A to 4A ). In these other molten metals  1 , the free crystals  6  (see  FIGS. 2A and 4A ) are not generated or the generation amount of the free crystal  6  (see  FIG. 3A ) becomes small. In response to this generation amount of the free crystal  6 , how a casting defect occurs changes. Moreover, the casting defect changes in accordance with a relationship between a surface of the casting mold and an equal solidification time curved surface of the molten metal  1 . The state in which the free crystal  6  is not generated includes a state in which the free crystal  6  is generated only in such an amount that the free crystal  6  cannot be used, in addition to a state in which the free crystal  6  is not generated at all. 
       FIGS. 5A to 8B  are diagrams for explaining occurrence of the casting defect  7  (here, a shrinkage cavity). In  FIGS. 5A  to  8 B, a part of a casting mold  10  is illustrated in sectional views, and the molten metal  1  and a cast-metal object  8  in the vicinity of a surface  11  of the casting mold  10  are schematically illustrated. The molten metal  1  is supplied (an arrow T in  FIGS. 5A to 8B ) from a riser (not shown) into the casting mold  10 . 
     As illustrated in  FIGS. 5A and 5B , and  FIGS. 6A and 6B , if the solidification direction S of the molten metal  1  is in parallel with the surface  11  of the casting mold  10 , the equal solidification time curved surface of the molten metal  1  is orthogonal to the surface  11  of the casting mold  10 . Solidification of the molten metal  1  proceeds along the surface  11  of the casting mold  10 , and the dendrite  5  grows in the molten metal  1 . As a result, the molten metal  1  is supplied in a gap of the dendrites  5  and thus, a shrinkage cavity is unlikely to occur in the cast-metal object  8  regardless of the type of the molten metal  1 . That is, when the molten metal  1  not generating the free crystal  6  is solidified, occurrence of the shrinkage cavity is suppressed (see  FIGS. 5A and 5B ) in the molten metal  1  with the narrow solidification temperature range K (see  FIG. 4B ) and the molten metal  1  with the wide solidification temperature range K (see  FIG. 2B ). Moreover, when the molten metal  1  generating the free crystal  6  is solidified, occurrence of a shrinkage cavity is suppressed (see  FIGS. 6A and 6B ) in the molten metal  1  with the narrow solidification temperature range K (see  FIG. 3B ) and the molten metal  1  with the wide solidification temperature range K (see  FIG. 1B ). 
     As illustrated in  FIGS. 7A and 7B , and  FIGS. 8A and 8B , if the solidification direction S of the molten metal  1  is orthogonal to the surface  11  of the casting mold  10 , the equal solidification time curved surface of the molten metal  1  becomes parallel with the surface  11  of the casting mold  10 . Solidification of the molten metal  1  proceeds toward the surface  11  of the casting mold  10 , and the dendrite  5  grows toward the surface  11  of the casting mold  10 . When the molten metal  1  is solidified to the surface  11  of the casting mold  10 , a cavity is formed on a surface of the cast-metal object  8  (see  FIGS. 7A and 7B ) due to solidification contraction of the molten metal  1 . The cavity of the cast-metal object  8  is a space closed by the surface  11  of the casting mold  10  and the dendrite  5 . Since the molten metal  1  is not replenished to the cavity of the cast-metal object  8 , a shrinkage cavity (casting defect  7 ) occurs in the cast-metal object  8 . At that time, in the molten metal  1  with the narrow solidification temperature range K (see  FIG. 4B ), the dendrite  5  is short, and thus a small shrinkage cavity occurs (see  FIG. 7A ). In the molten metal  1  with the wide solidification temperature range K (see  FIG. 2B ), the dendrite  5  is long, and thus a large shrinkage cavity occurs (See  FIG. 7B ). 
     On the other hand, in the molten metal  1  generating the free crystal  6  (see  FIGS. 8A and 8B ), while the molten metal  1  is solidified toward the surface  11  of the casting mold  10 , the free crystal  6  in the molten metal  1  is deposited on the surface  11  of the casting mold  10 . The free crystal  6  is deposited on the surface  11  until the solidification of the molten metal  1  reaches the surface  11  of the casting mold  10 , and the surface  11  is covered by the free crystal  6 . As a result, formation of a cavity in the cast-metal object  8  is prevented, and occurrence of the casting defect  7  (shrinkage cavity or the like) is suppressed. Moreover, deposition of the fine free crystals  6  generates a fine crystal structure on the surface of the cast-metal object  8 . 
     In the molten metal  1  with the narrow solidification temperature range K (see  FIG. 3B ), the generation amount of the free crystal  6  is small, and thus a deposited amount of the free crystal  6  becomes small (see  FIG. 8A ). In this case, the free crystal  6  is thinly deposited but since the free crystal  6  fills the gap of the short dendrites  5 , occurrence of the casting defect  7  is suppressed. In the molten metal  1  with the wide solidification temperature range K (see  FIG. 1B ), the generation amount of the free crystal  6  is large, and thus the deposited amount of the free crystal  6  increases, and the free crystal  6  is thickly deposited (see  FIG. 8B ). The gap of the dendrites  5  is reliably filled by the free crystal  6 , and occurrence of the casting defect  7  is suppressed. 
     As described above, in this embodiment, the free crystal  6  in the molten metal  1  is moved toward a predetermined surface  11  (deposited surface) of the casting mold  10  and is deposited on the surface  11 . The free crystal  6  is moved in the molten metal  1  and is deposited so as to cover the surface  11  (here, a design surface) of the casting mold  10 . As a result, the free crystal  6  is settled on the surface  11  of the casting mold  10 . In the casting mold  10 , the molten metal  1  is solidified toward the surface  11  of the casting mold  10  on which the free crystal  6  is deposited. For a period from start to end of solidification of the molten metal  1 , the free crystal  6  is generated in the molten metal  1  and is gradually deposited. However, if the free crystal  6  is present in the molten metal  1  before solidification of the molten metal  1  starts, this free crystal  6  is also deposited. In this case, the free crystal  6  that is deposited includes the free crystal  6  existing in the molten metal  1  in advance and the free crystal  6  generated in the molten metal  1 . 
     When the entire molten metal  1  is solidified, the cast-metal object  8  having a deposited portion  8 A of the free crystal  6  is produced. The deposited portion  8 A is a portion on which the free crystal  6  is deposited and includes a fine crystal structure. The deposited portion  8 A is formed on the surface of the cast-metal object  8  and covers the surface of the cast-metal object  8 . A portion of the cast-metal object  8  other than the deposited portion  8 A includes a normal crystal structure (columnar crystal and the like). 
     Subsequently, the process for producing the cast-metal object  8  of this embodiment will be specifically explained. 
       FIGS. 9A to 9C  are sectional views illustrating the process for producing the cast-metal object  8  of this embodiment.  FIGS. 9A to 9C  illustrate the producing process of the cast-metal object  8 . In  FIGS. 9A to 9C  (the same applies to each of the following figures), a portion (product portion) which is made into a product in the cast-metal object  8  is indicated by a two-dot chain line. 
     The casting mold  10  includes, as illustrated, a main mold  12  for molding the cast-metal object  8 , a molding flask  13  for accommodating the main mold  12 , a chiller  14 , and a supply pipe (stoke)  15  of the molten metal  1 . 
     The cast-metal object  8  which is made into a mold for tire molding is produced by the casting mold  10 . Here, the mold for tire molding is a mold for molding a tread for molding a tread portion of a tire and is formed of the cast-metal object  8  having a block shape. A plurality of molds for molding a tread is combined annularly and molds the tread portion of the tire. Thus, one surface of the cast-metal object  8  is formed into a shape corresponding to a tread pattern of the tire by the main mold  12 . The surface  11  of the main mold  12  is the design surface and has an irregular surface corresponding to the tread pattern. The cast-metal object  8  is molded into a predetermined shape by the main mold  12 . 
     A cavity  16  is formed in the casting mold  10  by the main mold  12  and the molding flask  13 . The chiller  14  is accommodated in the casting mold  10  and is arranged at a position in contact with the molten metal  1 . The cavity  16  is an internal space of the casting mold  10  and is formed into a shape corresponding to the shape of the cast-metal object  8 . The chiller  14  is arranged facing the main mold  12  and is exposed to the cavity  16 . The supply pipe  15  is made of a tubular insulating material and is provided on an upper part of the casting mold  10 . The molten metal  1  in the supply pipe  15  is a riser  1 A and is supplied to the cavity  16 . The cast-metal object  8  is produced by gravity casting by using the casting mold  10 . 
     When the cast-metal object  8  is to be produced, the metal material (an aluminum alloy, for example) is melted and the molten metal  1  is poured into the casting mold  10  (see  FIG. 9A ). The metal material is a metal material generating the free crystal  6  during solidification and is melted in a heating furnace (not shown). The molten metal  1  including a melted metal material generates the free crystal  6  along with solidification (see  FIG. 9B ). Solidification of the molten metal  1  starts from the chiller  14  and proceeds toward the main mold  12 . Thus, the solidification direction S of the molten metal  1  is a direction going from the chiller  14  toward the surface  11  of the casting mold  10  (the main mold  12 , here). The equal solidification time curved surface of the molten metal  1  moves in the solidification direction S. 
     While solidification of the molten metal  1  proceeds, the free crystal  6  is pushed out in the solidification direction S and is moved toward the surface  11  of the casting mold  10  located in the solidification direction S. As a result, while the free crystal  6  in the molten metal  1  is deposited on the surface  11  of the casting mold  10 , the molten metal  1  is solidified (see  FIG. 9C ). Moreover, by adjusting the solidification direction S and the position of the surface  11  of the casting mold  10 , the free crystal  6  is moved toward a predetermined portion of the surface  11  and is deposited on the predetermined portion. Here, the free crystal  6  is moved toward a portion where a shrinkage cavity is likely to occur and is deposited on the portion. Since the shrinkage cavity is likely to occur in a concave portion  12 A (groove or the like) of the main mold  12 , the free crystal  6  is deposited in the concave portion  12 A and is filled in the concave portion  12 A. As a result, occurrence of the shrinkage cavity in a protruding portion  8 B of the cast-metal object  8  is suppressed. At the same time, the deposited portion  8 A is formed on the protruding portion  8 B of the cast-metal object  8 , and a fine crystal structure is generated on the protruding portion  8 B. 
       FIGS. 10A to 10C  are sectional views illustrating the cast-metal object  8  in which the shrinkage cavity occurs.  FIGS. 10A to 10C  illustrate the producing process of the cast-metal object  8  in correspondence with  FIGS. 9A to 9C . 
     In the molten metal  1  not generating the free crystal  6 , the free crystal  6  is not deposited as illustrated and thus, the shrinkage cavity (casting defect  7 ) might occur in the cast-metal object  8 . The shrinkage cavity is formed in the closed space formed between the surface  11  of the casting mold  10  and the equal solidification time curved surface of the molten metal  1 . In this cast-metal object  8 , the shrinkage cavity occurs in the protruding portion  8 B. Moreover, since the deposited portion  8 A is not formed on the protruding portion  8 B, an effect of generating a fine crystal structure in the cast-metal object  8  is not obtained. 
     As described above, according to the process for producing the cast-metal object  8  of this embodiment, occurrence of the casting defect  7  in the cast-metal object  8  can be easily suppressed. Moreover, since the crystal structure of the cast-metal object  8  can be made fine, the mechanical property and strength of the cast-metal object  8  can be improved. As a result, the quality of the cast-metal object  8  can be improved. Since special labor is not needed before and after casting, time and labor for producing the cast-metal object  8  can be reduced. 
     When the cast-metal object  8  is to be produced by precision casting, too, occurrence of the casting defect  7  can be suppressed. Moreover, the cast-metal object  8  having a narrow and high protruding portion  8 B can be produced without the casting defect  7 . Thus, this process for producing the cast-metal object  8  is suitable for production of a mold for tire molding and can effectively improve the quality of the mold for tire molding. 
     Subsequently, a plurality of embodiments with different casting methods will be explained. In the following, the same configuration with the casting mold  10  already explained is given the same name and reference numerals and explanation thereof will be omitted. 
       FIGS. 11A to 11C  are sectional views illustrating a producing process of the cast-metal object  8  using the gravity. 
     In this casting mold  10 , as illustrated, the main mold  12  is installed on a lower part of the casting mold  10  and is arranged with the surface  11  of the main mold  12  directed upward. The chiller  14  is arranged above the main mold  12  so as to face the main mold  12 . The cavity  16  is formed between the main mold  12  and the chiller  14 . Two supply pipes  15  protrude upward from a side portion of the casting mold  10 . 
     In production of the cast-metal object  8 , a metal material generating the free crystal  6  is melted, and the molten metal is poured into the casting mold  10  (see  FIG. 11A ). Solidification of the molten metal  1  starts from the chiller  14  and proceeds toward the main mold  12  located below the chiller  14  (see  FIG. 11B ). The solidification direction S of the molten metal  1  is a direction (downward direction) going from the chiller  14  toward the surface  11  of the casting mold  10  (main mold  12 ). While solidification of the molten metal  1  proceeds, the free crystal  6  is pushed out in the solidification direction S and is settled in the molten metal  1  by the gravity. 
     The free crystal  6  is moved toward the surface  11  of the casting mold  10  located in the downward direction and is deposited on the surface  11 . At that time, the free crystal  6  in the molten metal  1  is settled toward a predetermined portion of the surface  11  of the casting mold  10  by the gravity and is deposited on the surface  11 . Moreover, the free crystal  6  is settled toward and deposited on the portion where the shrinkage cavity is likely to occur (the concave portion  12 A of the main mold  12 ). The free crystal  6  is deposited in the concave portion  12 A with priority and is filled in the concave portion  12 A. As described above, while the free crystal  6  in the molten metal  1  is deposited on the surface  11  of the casting mold  10 , the molten metal  1  is solidified (see  FIG. 11C ). Deposition of the free crystal  6  suppresses occurrence of the shrinkage cavity in the protruding portion  8 B of the cast-metal object  8 . The deposited portion  8 A is formed on the protruding portion  8 B of the cast-metal object  8 , and a fine crystal structure is generated. 
       FIGS. 12A to 12C  are sectional views illustrating the cast-metal object  8  in which the shrinkage cavity occurs.  FIGS. 12A to 12C  illustrate the producing process of the cast-metal object  8  in correspondence with  FIGS. 11A to 11C . 
     In the molten metal  1  not generating the free crystal  6 , settlement and deposition of the free crystal  6  are not generated as illustrated and thus, the shrinkage cavity (casting defect  7 ) might occur in the cast-metal object  8 . The shrinkage cavity occurs in the protruding portion  8 B of the cast-metal object  8  in the concave portion  12 A of the main mold  12 . Moreover, the deposited portion  8 A is not formed on the protruding portion  8 B of the cast-metal object  8 . 
     As described above, by making use of the free crystal  6 , occurrence of the casting defect  7  can be suppressed, and the crystal structure of the cast-metal object  8  can be made fine. Moreover, by settling the free crystal  6  toward the surface  11  of the casting mold  10  by the gravity, the free crystal  6  can be reliably deposited on the surface  11 . As a result, occurrence of the casting defect  7  can be reliably suppressed. By locating the portion where the casting defect  7  in the cast-metal object  8  occurs in the moving direction (settling direction) of the free crystal  6  by the gravity, occurrence of the casting defect  7  can be reliably suppressed. When the free crystal  6  is to be deposited, a physical force other than the gravity (a centrifugal force, an electromagnetic force, for example) may be used. 
       FIG. 13  is a sectional view illustrating the cast-metal object  8  produced by using a centrifugal force. The casting mold  10  illustrated in  FIG. 13  is the same as the casting mold  10  illustrated in  FIGS. 9A to 9C , and the cast-metal object  8  is produced by a process equivalent to the process explained in  FIGS. 9A to 9C . Thus, in  FIG. 13 , only the figure after the molten metal  1  is solidified is illustrated in correspondence with  FIG. 9C . 
     The casting mold  10  is fixed on a rotating table  21  of a rotating device  20  as illustrated. The rotating device  20  rotates the rotating table  21  by a driving device (not shown) (arrow R). As a result, the casting mold  10  into which the molten metal  1  is poured is rotated, and a centrifugal force is applied to the molten metal  1 . While solidification of the molten metal  1  proceeds, the free crystal  6  is pushed out in the solidification direction S and is flowed in the direction of the centrifugal force in the molten metal  1  by the centrifugal force. Since the main mold  12  is located in the direction of the centrifugal force in the rotating casting mold  10 , the free crystal  6  is flowed toward the main mold  12 . 
     The free crystal  6  is moved toward the surface  11  of the casting mold  10  (main mold  12 ) located in the direction of the centrifugal force and is deposited on the surface  11 . At that time, the free crystal  6  in the molten metal  1  is guided by the centrifugal force toward the predetermined portion of the surface  11  of the casting mold  10  and is deposited on the surface  11 . Moreover, the free crystal  6  is guided to the portion where the shrinkage cavity is likely to occur (the concave portion  12 A of the main mold  12 ) and is put in the concave portion  12 A. The free crystal  6  is deposited in the concave portion  12 A with priority and is filled in the concave portion  12 A. While the free crystal  6  in the molten metal  1  is deposited on the surface  11  of the casting mold  10 , the molten metal  1  is solidified. 
     As described above, by making use of the free crystal  6 , occurrence of the casting defect  7  can be suppressed, and the crystal structure of the cast-metal object  8  can be made fine. Moreover, by guiding the free crystal  6  toward the surface  11  of the casting mold  10  by the centrifugal force, the free crystal  6  can be reliably deposited on the surface  11 . As a result, occurrence of the casting defect  7  can be reliably suppressed. By locating the portion where the casting defect  7  in the cast-metal object  8  occurs in the moving direction (guiding direction) of the free crystal  6  by the centrifugal force, occurrence of the casting defect  7  can be reliably suppressed. 
       FIGS. 14A to 14C  are sectional views illustrating the producing process of the cast-metal object  8  by low-pressure casting. 
     The casting mold  10  is installed on an upper part of a supply device  30  of the molten metal  1  as illustrated. The supply device  30  includes a pressurized vessel  31 , a crucible  32  accommodated in the pressurized vessel  31 , and a heating device  33  for heating the crucible  32 . The molten metal  1  in the crucible  32  is heated to a predetermined temperature by the heating device  33 . The supply pipe  15  protrudes downward from the casting mold  10  and is arranged in the pressurized vessel  31  and the crucible  32 . The molten metal  1  is supplied through the supply pipe  15  into the casting mold  10  (cavity  16 ) from the crucible  32 . When a pressurizing device (not shown) supplies a pressurized gas G into the pressurized vessel  31 , the molten metal  1  in the crucible  32  is pressurized by the pressurized gas G. As a result, the molten metal  1  is pushed up through the supply pipe  15  and is poured into the casting mold  10 . 
     In production of the cast-metal object  8 , the metal material generating the free crystal  6  is melted, and the molten metal  1  is poured into the casting mold  10  by the supply device  30  (see  FIG. 14A ). Solidification of the molten metal  1  starts from the chiller  14  and proceeds toward the main mold  12  (see  FIG. 14B ). The chiller  14  is provided on an upper part and side parts of the casting mold  10  so as to surround the main mold  12 . Thus, the solidification direction S of the molten metal  1  is a direction going from a corner part of the chiller  14  toward the surface  11  of the casting mold  10  (main mold  12 ) (diagonally downward direction). Moreover, during solidification of the molten metal  1 , a flow F of the molten metal  1  is generated by convection. While solidification of the molten metal  1  proceeds, the free crystal  6  is pushed out in the solidification direction S and is flowed in the molten metal  1  by the flow F of the molten metal  1 . The free crystal  6  is moved toward the surface  11  of the casting mold  10  located in the direction in which the molten metal  1  is flowed and is deposited on the surface  11 . 
     In the casting mold  10 , the free crystal  6  in the molten metal  1  is flowed toward the surface  11  of the casting mold  10  by the flow F of the molten metal  1  and is deposited on the surface  11 . Moreover, by making adjustment so that the flow F of the molten metal  1  goes toward a predetermined portion of the surface  11 , the free crystal  6  is flowed toward the predetermined portion of the surface  11 . The free crystal  6  is carried to the surface  11  of the casting mold  10  carried by the flow F of the molten metal  1  and is deposited on the predetermined portion of the surface  11 . At that time, the free crystal  6  in the molten metal  1  is flowed toward a portion where the molten metal  1  is stagnant (stagnant portion) carried by the flow F of the molten metal  1  in the casting mold  10 . The free crystal  6  stays in the stagnant portion carried by the flow F of the molten metal  1 . As a result, the free crystal  6  is deposited on the surface  11  of the casting mold  10  where the molten metal  1  is stagnant. 
     The stagnant portion of the molten metal  1  is a portion where the shrinkage cavity is likely to occur, and the concave portion  12 A of the main mold  12  becomes the stagnant portion. The free crystal  6  is flowed toward the concave portion  12 A of the main mold  12  and is deposited in the concave portion  12 A with priority. As described above, while the free crystal  6  in the molten metal  1  is deposited on the surface  11  of the casting mold  10 , the molten metal  1  is solidified (see  FIG. 14C ). Deposition of the free crystal  6  suppresses occurrence of the shrinkage cavity in the protruding portion  8 B of the cast-metal object  8 . On the protruding portion  8 B of the cast-metal object  8 , the deposited portion  8 A is formed, and a fine crystal structure is generated. 
       FIGS. 15A to 15C  are sectional views illustrating the cast-metal object  8  in which the shrinkage cavity occurs.  FIGS. 15A to 15C  illustrate the producing process of the cast-metal object  8  in correspondence with  FIGS. 14A to 14C . 
     In the molten metal  1  not generating the free crystal  6 , the flow and deposition of the free crystal  6  are not generated as illustrated and thus, the shrinkage cavity (casting defect  7 ) might occur in the cast-metal object  8 . The shrinkage cavity occurs on the protruding portion  8 B of the cast-metal object  8  in the concave portion  12 A of the main mold  12 . Moreover, the deposited portion  8 A is not formed on the protruding portion  8 B of the cast-metal object  8 . 
     As described above, by making use of the free crystal  6 , occurrence of the casting defect  7  can be suppressed and the crystal structure of the cast-metal object  8  can be made fine. Moreover, by flowing the free crystal  6  toward the surface  11  of the casting mold  10  by the flow F of the molten metal  1 , the free crystal  6  can be reliably deposited on the surface  11 . As a result, occurrence of the casting defect  7  can be reliably suppressed. By carrying the free crystal  6  by the flow F of the molten metal  1  and depositing it on the surface  11  of the casting mold  10  where the molten metal  1  is stagnant, occurrence of the casting defect  7  can be suppressed more reliably. By locating the portion where the casting defect  7  in the cast-metal object  8  occurs in the moving direction (flowing direction) of the free crystal  6  by the flow F of the molten metal  1 , occurrence of the casting defect  7  can be reliably suppressed. 
     The free crystal  6  may be moved to the surface  11  of the casting mold  10  by using any one of the gravity, the centrifugal force, and the flow F of the molten metal  1 . Moreover, the free crystal  6  may be moved to the surface  11  of the casting mold by using arbitrary combination of the gravity, the centrifugal force, and the flow F of the molten metal  1 . 
     (Production Test of Cast-Metal Object  8 ) 
     In order to confirm the effect of the present invention, first to fourth production tests were conducted, and the cast-metal object  8  was produced. In each of the production tests, the molten metal  1  including the metal material generating the free crystal  6  is solidified in the casting mold  10  as described above, and the cast-metal object  8  (hereinafter referred to as an embodied product) was produced. Moreover, for comparison, the molten metal  1  including the metal material not generating the free crystal  6  is solidified in the casting mold  10 , and the cast-metal object  8  (hereinafter referred to as a comparative product) was produced. The embodied product and the comparative product were inspected, and occurrence situations of the casting defect  7  were investigated. 
       FIGS. 16A to 16C  are views illustrating the cast-metal object  8  produced by the casting mold  10 .  FIG. 16A  is a front view of the cast-metal object  8 .  FIG. 16B  is a top view of the cast-metal object  8  when seen from an X direction of  FIG. 16A .  FIG. 16C  is a side view of the cast-metal object  8  when seen from a Y direction of  FIG. 16A . 
     The cast-metal object  8  is a mold for tire molding as illustrated and is formed into a block shape. After the cast-metal object  8  is taken out of the casting mold  10 , the mold for tire molding is formed by working the cast-metal object  8 . Here, the cast-metal object  8  is a mold for tread molding (sector segment) and has a plurality of protruding portions  8 B for forming grooves in a tire. 
     Production conditions of the cast-metal objects  8  (embodied product, comparative product) are as follows: 
     Metal material of the embodied product: JIS casting aluminum alloy (AC2B) (Si: 6%, Cu: 2.5%, Mg: 0.4%, Fe: 0.5%, Zn: 0.2%, Bal: Al) 
     Metal material of the comparative product: JIS casting aluminum alloy (AC4C) (Si: 7%, Mg: 0.4%, Fe: 0.3%, Bal: Al) 
     Each of the metal materials was melted, and the molten metal  1  (690° C.) was poured into the casting mold  10 . 
     Materials of each portion of the casting mold  10  are as follows: 
     Main mold  12 : non-foaming plaster (by Noritake Co., Ltd., product name: G-6) 
     Chiller  14 : gray cast iron (JIS standard product, FC 250) 
     Molding flask  13 : insulating material (calcium silicate) 
     Supply pipe  15 : insulating material (alumina ceramic) 
     The nine embodied products and the nine comparative products were cast by four types of the casting molds  10  illustrated below. At that time, the chiller  14  was pre-heated to a temperature of 250 to 300° C. 
       FIGS. 17A to 17C  are sectional views of the casting mold  10  used in the first production test.  FIG. 17A  illustrates the casting mold  10  when seen from the front.  FIG. 17B  illustrates the casting mold  10  when seen from the X-direction of  FIG. 17A .  FIG. 17C  illustrates the casting mold  10  when seen from the Y-direction of  FIG. 17A . 
     As illustrated, the casting mold  10  of the first production test was configured similarly to the casting mold illustrated in  FIGS. 9A to 9C . The main mold  12  is accommodated in the box-shaped chiller  14  and is arranged on the side part in the casting mold  10 . 
     During casting, a height of the riser  1 A was set to 300 mm, and the cast-metal object  8  was produced by gravity casting. In the comparative products in the first production test, slight shrinkage cavities occurred in seven cast-metal objects  8 . The shrinkage cavities occurred in a Z portion (the protruding portion  8 B or a periphery of the protruding portion  8 B). On the other hand, in the embodied products, extremely slight shrinkage cavities occurred in the Z portions of two cast-metal objects  8 . 
       FIGS. 18A to 18C  are sectional views of the casting mold  10  used in the second production test.  FIG. 18A  illustrates the casting mold  10  when seen from the front.  FIG. 18B  illustrates the casting mold  10  when seen from the X-direction of  FIG. 18A .  FIG. 18C  illustrates the casting mold  10  when seen from the Y-direction of  FIG. 18A . 
     As illustrated, the casting mold  10  of the second production test was configured similarly to the casting mold  10  illustrated in  FIGS. 11A to 11C . The main mold  12  is accommodated in the box-shaped chiller  14  and is arranged on the lower part in the casting mold  10 . Above the main mold  12 , the supply pipe  15  and the chiller  14  were arranged. 
     During casting, the height of the riser  1 A was set to 300 mm, and the cast-metal object  8  was produced by gravity casting. In the comparative products in the second production test, slight shrinkage cavities occurred in the Z portions of the four cast-metal objects  8 . On the other hand, when the embodied product was to be produced, the free crystal  6  was settled toward the surface  11  of the casting mold  10  (main mold  12 ) by the gravity and was deposited thereon during solidification of the molten metal  1 . As a result, in the embodied products, extremely slight shrinkage cavities occurred in the Z portion of one cast-metal object  8 . 
       FIGS. 19A to 19C  are sectional views of the casting mold  10  used in the third production test.  FIG. 19A  illustrates the casting mold  10  when seen from the front.  FIG. 19B  illustrates the casting mold  10  when seen from the X-direction of  FIG. 19A .  FIG. 19C  illustrates the casting mold  10  when seen from the Y-direction of  FIG. 19A . 
     As illustrated, the casting mold  10  of the third production test was configured similarly to the casting mold  10  illustrated in  FIGS. 14A to 14C . The main mold  12  is accommodated in the box-shaped chiller  14  and is arranged on the side part in the casting mold  10 . 
     During casting, the molten metal  1  was supplied into the casting mold  10  by the supply device  30  (not shown in  FIGS. 19A to 19C ), and the cast-metal object  8  was produced by low-pressure casting. At that time, the molten metal  1  was pressurized by the supply device  30  with a predetermined pressure (atmospheric pressure+0.05 MPa). In the comparative products in the third production test, slight shrinkage cavities occurred in the Z portions of the four cast-metal objects  8 . On the other hand, when the embodied product was to be produced, the free crystal  6  was flowed toward the surface  11  of the casting mold  10  (main mold  12 ) by the above-described flow F of the molten metal  1  and was deposited thereon during solidification of the molten metal  1 . Moreover, the free crystal  6  was deposited on the stagnant portion of the molten metal  1  carried by the flow F of the molten metal  1 . As a result, in the embodied products, extremely slight shrinkage cavities occurred in the Z portion of one cast-metal object  8 . 
       FIGS. 20A to 20C  are sectional views of the casting mold  10  used in the fourth production test.  FIG. 20A  illustrates the casting mold  10  when seen from the front.  FIG. 20B  illustrates the casting mold  10  when seen from the X-direction of  FIG. 20A .  FIG. 20C  illustrates the casting mold  10  when seen from the Y-direction of  FIG. 20A . 
     As illustrated, the casting mold  10  of the fourth production test was configured similarly to the casting mold  10  illustrated in  FIGS. 14A to 14C . The main mold  12  is accommodated in the box-shaped chiller  14  and is arranged on the lower part of the chiller  14 . The molten metal  1  is poured from below the main mold  12  into the casting mold  10  and supplied to the cavity  16  located above the main mold  12 . 
     During casting, the molten metal  1  was supplied into the casting mold  10  by the supply device  30  (not shown in  FIGS. 20A to 20C ), and the cast-metal object  8  was produced by low-pressure casting. At that time, the molten metal  1  was pressurized by the supply device  30  with the predetermined pressure (atmospheric pressure+0.05 MPa). In the comparative products in the fourth production test, slight shrinkage cavities occurred in the Z portions of the four cast-metal objects  8 . On the other hand, when the embodied product was to be produced, the free crystal  6  was settled toward the surface  11  of the casting mold  10  (main mold  12 ) by the gravity and was deposited thereon during solidification of the molten metal  1 . At the same time, the free crystal  6  was flowed toward the surface  11  of the casting mold  10  by the above-described flow F of the molten metal  1  and was deposited thereon. Moreover, the free crystal  6  was deposited on the stagnant portion of the molten metal  1  carried by the flow F of the molten metal  1 . As a result, in the embodied products, shrinkage cavities did not occur in any of the cast-metal objects  8 . 
     As described above, in the embodied products, it was confirmed that occurrence of shrinkage cavities was drastically fewer than the comparative products. Therefore, it was proved that occurrence of the casting defect  7  in the cast-metal object  8  can be suppressed by the present invention. Moreover, the crystal structure of the embodied product (deposited portion  8 A) was found to be finer than the crystal structure of the comparative product (portion corresponding to the deposited portion  8 A). 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  molten metal 
               1 A riser 
               2  solid phase 
               3  liquid phase 
               4  solid-liquid coexisting region 
               5  dendrite 
               6  free crystal 
               7  casting defect 
               8  cast-metal object 
               8 A deposited portion 
               8 B protruding portion 
               10  casting mold 
               11  surface 
               12  main mold 
               12 A concave portion 
               13  molding flask 
               14  chiller 
               15  supply pipe 
               16  cavity 
               20  rotating device 
               21  rotating table 
               30  supply device 
               31  pressurized vessel 
               32  crucible 
               33  heating device 
             G pressurized gas 
             K solidification temperature range 
             S solidification direction