Patent Publication Number: US-9427797-B2

Title: Up-drawing continuous casting apparatus and up-drawing continuous casting method

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2014-046046 filed on Mar. 10, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an up-drawing continuous casting apparatus and an up-drawing continuous casting method. 
     2. Description of Related Art 
     Japanese Patent Application Publication No. 2012-61518 (JP 2012-61518 A) proposes a free casting method as a groundbreaking up-drawing continuous casting method that does not require a mold. As described in JP 2012-61518 A, a starter is first dipped into the surface of molten metal (a molten metal surface), and then when the starter is drawn up, molten metal is also drawn up following the starter by surface tension and the surface film of the molten metal. Here, a casting that has a desired sectional shape is able to be continuously cast by drawing up the molten metal through a shape determining member arranged near the molten metal surface, and cooling the drawn up molten metal. 
     With a normal continuous casting method, the sectional shape and the shape in the longitudinal direction are both determined by a mold. In particular, with a continuous casting method, the solidified metal (i.e., the casting) must pass through the mold, so the cast casting takes on a shape that extends linearly in the longitudinal direction. In contrast, the shape determining member in the free casting method determines only the sectional shape of the casting. The shape in the longitudinal direction is not determined. Therefore, castings of various shapes in the longitudinal direction are able to be obtained by drawing the starter up while moving the starter (or the shape determining member) in a horizontal direction. For example, JP 2012-61518 A describes a hollow casting (i.e., a pipe) formed in a zigzag shape or a helical shape, not a linear shape in the longitudinal direction. 
     The inventors discovered the problem described below. With the free casting method described in JP 2012-61518 A, the molten metal drawn up through the shape determining member is cooled and solidified by cooling gas, so a solidification interface is positioned above the shape determining member. The position of this solidification interface directly affects the dimensional accuracy and surface quality of the casting. Therefore, it is essential to detect the solidification interface and control it to within a predetermined reference range. 
     Here, the inventors have found that, because the surface of the drawn-up molten metal oscillates (more specifically, greatly fluctuates in short fluctuation cycles) and the surface of the casting formed by the molten metal solidifying does not oscillate much at all (more specifically, fluctuates little in long fluctuation cycles), the solidification interface can be determined based on whether there is oscillation. However, if the position of the solidification interface is low, oscillation of the drawn-up molten metal is small and is difficult to detect, so it is difficult to determine the solidification interface based on whether there is oscillation. As a result, if the position of the solidification interface is low, the solidification interface may not be able to be controlled to within an appropriate reference range. 
     SUMMARY OF THE INVENTION 
     The invention thus provides an up-drawing continuous casting apparatus and an up-drawing continuous casting method in which a solidification interface can be controlled to within an appropriate reference range even if the solidification interface is low, and which therefore obtain excellent dimensional accuracy and surface quality of a casting. 
     A first aspect of the invention relates to an up-drawing continuous casting apparatus that includes a holding furnace that holds molten metal; a shape determining member that is arranged above a molten metal surface of the molten metal held in the holding furnace, and that determines a sectional shape of a cast casting by the molten metal passing through the shape determining member, the shape determining member including a pattern provided on an upper surface of the shape determining member; an imaging portion configured to capture an image of the pattern that is reflected onto both retained molten metal that has passed through the shape determining member, and the casting formed by the retained molten metal solidifying; an image analyzing portion configured to determine a solidification interface from the image; and a casting controlling portion configured to change a casting condition when the solidification interface determined by the image analyzing portion is not within a predetermined reference range. With the up-drawing continuous casting apparatus according to this first aspect of the invention, the pattern provided on the upper surface of the solidification interface is reflected onto the molten metal that has passed through the shape determining member, so the brightness of the molten metal surface greatly changes with even the slightest oscillation of the molten metal. Therefore, the solidification interface is able to be determined even if the solidification interface is low and the oscillation of the molten metal is small. As a result, the solidification interface is able to be controlled to within an appropriate reference range even if the solidification interface is low. 
     A second aspect of the invention relates to an up-drawing continuous casting method that includes arranging a shape determining member that determines a sectional shape of a cast casting above a molten metal surface of molten metal held in a holding furnace, and drawing up the molten metal while passing the molten metal through the shape determining member, the shape determining member including a pattern provided on an upper surface of the shape determining member. This up-drawing continuous casting method also includes capturing an image of the pattern that is reflected onto both retained molten metal that has passed through the shape determining member, and the casting formed by the retained molten metal solidifying; determining a solidification interface from the image; and changing a casting condition when the determined solidification interface is not within a predetermined reference range. With the up-drawing continuous casting method according to this second aspect of the invention, the pattern provided on the upper surface of the solidification interface is reflected onto the molten metal that has passed through the shape determining member, so the brightness of the molten metal surface greatly changes with even the slightest oscillation of the molten metal. Therefore, the solidification interface is able to be determined even if the solidification interface is low and the oscillation of the molten metal is small. As a result, the solidification interface is able to be controlled to within an appropriate reference range even if the solidification interface is low. 
     The invention is thus able to provide an up-drawing continuous casting apparatus and an up-drawing continuous casting method in which a solidification interface can be controlled to within an appropriate reference range even if the solidification interface is low, and which therefore obtain excellent dimensional accuracy and surface quality of a casting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a sectional view showing a frame format of a free casting apparatus according to a first example embodiment of the invention; 
         FIG. 2  is a plan view of a shape determining member according to the first example embodiment; 
         FIG. 3  is a block diagram of a solidification interface control system provided in the free casting apparatus according to the first example embodiment; 
         FIG. 4  is a view of three example images of an area near a solidification interface; 
         FIG. 5  is a flowchart illustrating a solidification interface control method according to the first example embodiment; 
         FIG. 6  is a plan view of a modified example of the shape determining member according to the first example embodiment; 
         FIG. 7  is a plan view of the modified example of the shape determining member according to the first example embodiment; 
         FIG. 8  is a side view of the modified example of the shape determining member according to the first example embodiment; 
         FIG. 9  is a view of an image of the shape determining member used in a test; 
         FIG. 10  is a view of example images of an area near the solidification interface in a case in which a pattern is not applied to an upper surface of the shape determining member, and a case in which the pattern is applied to the upper surface of the shape determining member; 
         FIG. 11  is a view illustrating a test method; 
         FIG. 12  is a view of the relationship between the position of the solidification interface and interface detection rate; 
         FIG. 13  is a plan view of a shape determining member according to a second example embodiment of the invention; 
         FIG. 14  is a side view of the shape determining member of the second example embodiment; and 
         FIG. 15  is a flowchart illustrating a solidification interface control method according to the second example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, specific example embodiments to which the invention has been applied will be described in detail with reference to the accompanying drawings. However, the invention is not limited to these example embodiments. Also, the description and the drawings are simplified as appropriate to clarify the description. 
     First Example Embodiment 
     First, a free casting apparatus (up-drawing continuous casting apparatus) according to a first example embodiment of the invention will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view showing a frame format of the free casting apparatus according to the first example embodiment. As shown in  FIG. 1 , the free casting apparatus according to the first example embodiment includes a molten metal holding furnace  101 , a shape determining member  102 , a support rod  104 , an actuator  105 , a cooling gas nozzle  106 , a cooling gas supplying portion  107 , an up-drawing machine  108 , and an imaging portion (camera)  109 . In  FIG. 1 , a right-handed xyz coordinate system is shown for descriptive purposes to illustrate the positional relationship of the constituent elements. The x-y plane in  FIG. 1  forms a horizontal plane, and the z-axis direction is the vertical direction. More specifically, the plus direction of the z-axis is vertically upward. 
     The molten metal holding furnace  101  holds molten metal M 1  such as aluminum or an aluminum alloy, for example, and keeps it at a predetermined temperature at which the molten metal M 1  has fluidity. In the example in  FIG. 1 , molten metal is not replenished into the molten metal holding furnace  101  during casting, so the surface of the molten metal M 1  (i.e., the molten metal surface level) drops as casting proceeds. However, molten metal may also be replenished into the molten metal holding furnace  101  when necessary during casting so that the molten metal surface level is kept constant. Here, the position of a solidification interface SIF can be raised by increasing a set temperature of the molten metal holding furnace  101 , and lowered by reducing the set temperature of the molten metal holding furnace  101 . Naturally, the molten metal M 1  may be another metal or alloy other than aluminum. 
     The shape determining member  102  is made of ceramic or stainless steel, for example, and is arranged above the molten metal M 1 . The shape determining member  102  determines the sectional shape of a cast casting M 3 . The casting M 3  shown in  FIG. 1  is a solid casting (a plate) having a rectangular cross-section in the horizontal direction (hereinafter, simply referred to as “transverse section”). Naturally, the sectional shape of the casting M 3  is not particularly limited. The casting M 3  may also be a hollow casting of a round pipe or a square pipe or the like. 
     In the example in  FIG. 1 , a main surface (a lower surface) on a lower side of the shape determining member  102  is arranged contacting the molten metal surface. Therefore, an oxide film that forms on the surface of the molten metal M 1  and foreign matter floating on the surface of the molten metal M 1  are able to be prevented from getting mixed into the casting M 3 . However, the lower surface of the shape determining member  102  may also be arranged a predetermined distance away from the molten metal surface. When the shape determining member  102  is arranged away from the molten metal surface, heat deformation and erosion of the shape determining member  102  are inhibited, so the durability of the shape determining member  102  improves. 
       FIG. 2  is a plan view of the shape determining member  102  according to the first example embodiment. Here, the sectional view of the shape determining member  102  in  FIG. 1  corresponds to a sectional view taken along line I-I in  FIG. 2 . As shown in  FIG. 2 , the shape determining member  102  has a rectangular planar shape, for example, and has a rectangular open portion (a molten metal passage portion  103 ) having a thickness t1 and a width w1 through which the molten metal passes in the center portion. The xyz coordinates in  FIG. 2  match those in  FIG. 1 . 
     Furthermore, a pattern P is applied to an upper surface (i.e., the surface on the upper side) of the shape determining member  102 . More specifically, a striped pattern P formed by a plurality of colors (black and white in this case) is applied to the upper surface of the shape determining member  102 . The pattern P is preferably applied such that the pattern P has slimness (density) where the colors are enough to be able to be identified by an image analyzing portion  110 . The pattern P is applied by applying heat resistance ink to the upper surface of the shape determining member  102 , for example. The specific effects of the pattern P will be described later. 
     As shown in  FIG. 1 , after joining with a starter ST that has been dipped into the molten metal M 1 , the molten metal M 1  is drawn up following the starter ST while maintaining its outer shape, by the surface tension and the surface film of the molten metal M 1 , and passes through the molten metal passage portion  103  of the shape determining member  102 . By passing the molten metal M 1  through the molten metal passage portion  103  of the shape determining member  102 , external force is applied to the molten metal M 1  from the shape determining member  102 , such that the sectional shape of the casting M 3  is determined. Here, the molten metal that is drawn up from the molten metal surface following the starter ST (or the casting M 3  that is formed by the molten metal M 1  drawn up following the starter ST solidifying) by the surface tension and the surface film of the molten metal M 1  will be referred to as “retained molten metal M 2 ”. Also, the boundary between the casting M 3  and the retained molten metal M 2  is a solidification interface SIF. 
     The support rod  104  supports the shape determining member  102 . The support rod  104  is connected to the actuator  105 . The shape determining member  102  is able to move up and down (i.e., in the vertical direction; the z-axis direction) via the support rod  104 , by the actuator  105 . According to this kind of structure, the shape determining member  102  is able to be moved downward as the molten metal surface level drops as casting proceeds. 
     A cooling gas nozzle (a cooling portion)  106  is cooling means for spraying cooling gas (e.g., air, nitrogen, argon, or the like) supplied from the cooling gas supplying portion  107  at the casting M 3  to cool the casting M 3 . The position of the solidification interface SIF is able to be lowered by increasing the flow rate of the cooling gas, and raised by reducing the flow rate of the cooling gas. The cooling gas nozzle  106  is also able to be moved up and down (i.e., in the vertical direction; in the z-axis direction) and horizontally (i.e., in the x-axis direction and the y-axis direction). Therefore, for example, the cooling gas nozzle  106  can be moved downward, in concert with the movement of the shape determining member  102 , as the molten metal surface level drops as casting proceeds. Alternatively, the cooling gas nozzle  106  can be moved horizontally, in concert with horizontal movement of the up-drawing machine  108 . 
     The casting M 3  is formed by the retained molten metal M 2  near the solidification interface SIF progressively solidifying from the upper side (i.e., a plus side in the z-axis direction) toward lower side (i.e., a minus side in the z-axis direction), by cooling the starter ST and the casting M 3  with the cooling gas, while drawing the casting M 3  up with the up-drawing machine  108  that is connected to the starter ST. The position of the solidification interface SIF is able to be raised by increasing the up-drawing speed with the up-drawing machine  108 , and lowered by reducing the up-drawing speed. Also, the retained molten metal M 2  is able to be drawn out diagonally by drawing the casting M 3  up while moving the up-drawing machine  108  horizontally (in the x-axis direction and the y-axis direction). Therefore, the longitudinal shape of the casting M 3  is able to be freely changed. The longitudinal shape of the casting M 3  may also be freely changed by moving the shape determining member  102  horizontally, instead of by moving the up-drawing machine  108  horizontally. 
     The imaging portion  109  continuously monitors the area near the solidification interface SIF that is the boundary between the casting M 3  and the retained molten metal M 2 , during casting. Here, the imaging portion  109  is arranged at a position and angle such that it is able to capture the pattern P reflected onto the surfaces of both the retained molten metal M 2  and the casting M 3  (or more preferably, the entire area used for image analysis). Also, the pattern P is applied to a position and area that satisfies this. As a result, the imaging portion  109  successively captures an image of not only the surfaces of both the retained molten metal M 2  and the casting M 3 , but also of the pattern P reflected onto these surfaces. In the example in  FIG. 1 , the imaging portion  109  is arranged looking diagonally down and facing on the solidification interface SIF from above the solidification interface SIF. When it is known in advance that the position of the solidification interface SIF will change, the imaging portion  109  may also be configured to move according to this change. The solidification interface SIF is able to be determined from the image captured by the imaging portion  109 , as will be described in detail later. 
     Next, a solidification interface control system provided in the free casting apparatus according to the first example embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a block diagram of the solidification interface control system provided in the free casting apparatus according to the first example embodiment. This solidification interface control system is designed to keep the position (height) of the solidification interface SIF within a predetermined reference range. 
     As shown in  FIG. 3 , this solidification interface control system includes the imaging portion  109 , an image analyzing portion  110 , a casting controlling portion  111 , the up-drawing machine  108 , the molten metal holding furnace  101 , and the cooling gas supplying portion  107 . The imaging portion  109 , the up-drawing machine  108 , the molten metal holding furnace  101 , and the cooling gas supplying portion  107  have been described with reference to  FIG. 1 , so detailed descriptions of these will be omitted here. 
     The image analyzing portion  110  determines the solidification interface from an image captured by the imaging portion  109 . More specifically, the image analyzing portion  110  compares a plurality of images captured in succession, and determines a location where a brightness value of reflected light changes greatly in short fluctuation cycles, to be the surface of the retained molten metal M 2  which oscillates. On the other hand, the image analyzing portion  110  determines a location where the brightness value of the reflected light changes only slightly in long fluctuation cycles, i.e., a location where there is not much oscillation, to be the surface of the casting M 3 . As a result, the image analyzing portion  110  is able to determine the solidification interface based on whether there is oscillation (or more specifically, the fluctuation cycle of the oscillation and fluctuation range of the oscillation). 
     Here, as described above, the pattern P is applied to the upper surface of the shape determining member  102 . This pattern P is reflected onto the retained molten metal M 2 , so the brightness of the surface of the retained molten metal M 2  changes greatly when the retained molten metal M 2  oscillates slightly. Therefore, the solidification interface is able to be determined even when the molten metal surface is low and oscillation of the molten metal surface is small. 
     This will be described in more detail with reference to  FIG. 4 .  FIG. 4  is a view of three example images of the area near the solidification interface. The example images in  FIG. 4  are, in order from the top of  FIG. 4 , an example image of a case in which the position of the solidification interface is above an upper limit, an example image of a case in which the position of the solidification interface is within the reference range, and an example image of a case in which the position of the solidification interface is below a lower limit As shown in the example image in the center of  FIG. 4 , the image analyzing portion  110  determines a boundary portion between a region where oscillation is detected (i.e., molten metal), and a region where oscillation is so small that it is not detected (i.e., the casting), in the image captured by the imaging portion  109 , to be the solidification interface, for example. 
     The casting controlling portion  111  includes a storing portion, not shown, that stores the reference range (the upper and lower limits) of the solidification interface position. Also, if the solidification interface determined by the image analyzing portion  110  is above the upper limit, the casting controlling portion  111  reduces the up-drawing speed of the up-drawing machine  108 , lowers the set temperature of the molten metal holding furnace  101 , or increases the flow rate of the cooling gas supplied from the cooling gas supplying portion  107 . On the other hand, if the solidification interface determined by the image analyzing portion  110  is below the lower limit, the casting controlling portion  111  increases the up-drawing speed of the up-drawing machine  108 , raises the set temperature of the molten metal holding furnace  101 , or decreases the flow rate of the cooling gas supplied from the cooling gas supplying portion  107 . Control of these three conditions may simultaneously change two or more conditions, but changing only one condition makes control easier, and is thus preferable. Also, the priority order of the three conditions may be set in advance, and they may be changed in order from that of the highest priority. 
     Next, the upper and lower limits of the solidification interface position will be described with reference to  FIG. 4 . As shown in the example images in  FIG. 4 , when the position of the solidification interface is above the upper limit, a “constriction” occurs in the retained molten metal M 2  and develops into a “tear”. The upper limit of the solidification interface position can be determined by changing the height of the solidification interface, and examining in advance whether a “constriction” occurs in the retained molten metal M 2 . 
     On the other hand, when the position of the solidification interface is below the lower limit, as shown in the example image at the bottom of  FIG. 4 , asperities occur on the surface of the casting M 3  and become shape defects. The lower limit of the solidification interface position can be determined by changing the height of the solidification interface, and examining in advance whether asperities occur on the surface of the casting M 3 . These asperities are thought to be solidified flakes that have formed inside the shape determining member  102  due to the solidification interface being too low. 
     In this way, the free casting apparatus according to the first example embodiment has the pattern P applied to the upper surface of the shape determining member  102 , and includes the imaging portion that captures an image of the pattern P that is reflected onto an area near the solidification interface, and an image analyzing portion that determines the solidification interface from this image. Because this pattern P is reflected onto the retained molten metal M 2 , the brightness of the surface of the retained molten metal M 2  greatly changes when the retained molten metal M 2  oscillates slightly. Therefore, the solidification interface is able to be determined even if the solidification interface is low and the oscillation of the molten metal is small. As a result, even if the solidification interface is low, feedback control for keeping the solidification interface within the predetermined reference range is able to be performed, so the dimensional accuracy and surface quality of the casting are able to be improved. 
     Continuing on, a free casting method according to the first example embodiment will be described with reference to  FIG. 1 . 
     First, the starter ST is lowered by the up-drawing machine  108  so that it passes through the molten metal passage portion  103  of the shape determining member  102 , and the tip end portion of the starter ST is dipped into the molten metal M 1 . 
     Next, the starter ST starts to be drawn up at a predetermined speed. Here, even if the starter ST separates from the molten metal surface, the molten metal M 1  follows the starter ST and is drawn up from the molten metal surface by the surface film and surface tension, and forms the retained molten metal M 2 . As shown in  FIG. 1 , the retained molten metal M 2  is formed in the molten metal passage portion  103  of the shape determining member  102 . That is, the shape determining member  102  gives the retained molten metal M 2  its shape. 
     Next, the starter ST (or the casting M 3  formed by the retained molten metal M 2  solidifying) is cooled by cooling gas blown from the cooling gas nozzle  106 . As a result, the retained molten metal M 2  is indirectly cooled and solidifies progressively from the upper side toward the lower side, thus forming the casting M 3 . In this way, the casting M 3  is able to be continuously cast. 
     The free casting method according to the first example embodiment controls the solidification interface so as to keep it within a predetermined reference range. Hereinafter, the solidification interface control method will be described with reference to  FIG. 5 .  FIG. 5  is a flowchart illustrating the solidification interface control method according to the first example embodiment. 
     First, the imaging portion  109  captures an image of the area near the solidification interface (step ST 1 ). Then, the image analyzing portion  110  analyzes the image captured by the imaging portion  109  (step ST 2 ). More specifically, the image analyzing portion  110  determines a location where the brightness value of reflected light changes greatly in short fluctuation cycles, to be the surface of the retained molten metal M 2  which oscillates, and determines a location where there is almost no oscillation to be the surface of the casting M 3 , by comparing a plurality of images captured in succession. Then the image analyzing portion  110  determines the boundary portion between a region where oscillation was detected and a region where oscillation was so small that it was not detected, in the image captured by the imaging portion  109 , to be the solidification interface. 
     Here, the pattern P is applied to the upper surface of the shape determining member  102 . This pattern P is reflected onto the retained molten metal M 2 , so the brightness of the surface of the retained molten metal M 2  changes greatly when the retained molten metal M 2  oscillates slightly. Therefore, the solidification interface is able to be determined even when the molten metal surface is low and the oscillation of the molten metal surface is small. 
     Next, the casting controlling portion  111  determines whether the position of the solidification interface determined by the image analyzing portion  110  is within the reference range (step ST 3 ). If the position of the solidification interface is not within the reference range (i.e., NO in step ST 3 ), the casting controlling portion  111  changes one of the conditions, i.e., the cooling gas flow rate, the casting speed, and the holding furnace set temperature (step ST 4 ). Then, the casting controlling portion  111  determines whether casting is complete (step ST 5 ). 
     More specifically, in step ST 4 , if the solidification interface determined by the image analyzing portion  110  is above the upper limit, the casting controlling portion  111  reduces the up-drawing speed of the up-drawing machine  108 , lowers the set temperature of the molten metal holding furnace  101 , or increases the flow rate of cooling gas supplied from the cooling gas supplying portion  107 . On the other hand, if the solidification interface determined by the image analyzing portion  110  is below the lower limit, the casting controlling portion  111  increases the up-drawing speed of the up-drawing machine  108 , raises the set temperature of the molten metal holding furnace  101 , or reduces the flow rate of the cooling gas supplied from the cooling gas supplying portion  107 . 
     If the position of the solidification interface is within the reference range (i.e., YES in step ST 3 ), none of the casting conditions are changed and the process proceeds directly on to step ST 5 . 
     If casting is not complete (i.e., NO in step ST 5 ), the process returns to step ST 1 . On the other hand, if casting is complete (i.e., YES in step ST 5 ), control of the solidification interface ends. 
     In this way, with the free casting method according to the first example embodiment, the pattern P is applied to the upper surface of the shape determining member  102 , and an image of the pattern P reflected onto an area near the solidification interface is captured, and the solidification interface is determined from this image. Because this pattern P is reflected onto the retained molten metal M 2 , the brightness of the surface of the retained molten metal M 2  changes greatly when the retained molten metal M 2  oscillates slightly. Therefore, the solidification interface is able to be determined even if the solidification interface is low and oscillation of the molten metal is small. As a result, even if the solidification interface is low, feedback control for keeping the solidification interface within the predetermined reference range is able to be performed, so the dimensional accuracy and surface quality of the casting are able to be improved. 
     In this example embodiment, the pattern P is described as being made up of black and white colors, but it is not limited to this. The pattern P may be made up of any two or more suitable colors. Also, in this example embodiment, an example in which the pattern P is striped is described, but the pattern P is not limited to this. The pattern P may be a pattern of any suitable shape, e.g., a mesh shape such as that shown in  FIG. 6 . 
     Alternatively, the pattern P may be formed by applying a serrated shape to the upper surface of the shape determining member  102 , as shown in the plan view of  FIG. 7  and the side view of  FIG. 8 . As a result, different brightnesses are able to be distributed onto the upper surface of the shape determining member  102 , so the brightness of the surface of the retained molten metal M 2  is able to be greatly changed by even the slightest oscillation of the retained molten metal M 2 , just as with the case in which the pattern P is formed by a plurality of colors. Therefore, the solidification interface is able to be determined even if the solidification interface is low and oscillation of the molten metal is small. 
     (Test Results) 
     Continuing on, the inventors changed the height of the solidification interface and measured an interface detection rate, so the test results from this will now be described. Here, the interface detection rate is the ratio of the time for which the image analyzing portion  110  was able to detect the solidification interface to the capturing time by the imaging portion  109 . 
     In this test, the interface detection rate was measured for a case in which the pattern P was not applied to the upper surface of the shape determining member  102 , and a case in which a mesh-shaped pattern P such as that shown in  FIG. 9  was applied to the upper surface of the shape determining member  102 .  FIG. 10  is a view of example images of an area near the solidification interface in a case in which the pattern P was not applied to the upper surface of the shape determining member  102 , and a case in which the pattern P was applied to the upper surface of the shape determining member  102 . With the case in which the pattern P was applied, it is evident that the pattern P is reflected onto the retained molten metal M 2 , as shown in  FIG. 10 . 
       FIG. 11  is a view illustrating the test method. The xyz coordinates in  FIG. 11  are the same as those in  FIG. 1 . In this test, the imaging portion  109  is arranged so as to capture an image of the minus side from the x-axis direction plus side, as shown in  FIG. 11 . 
     First, at time t1 to t2, the molten metal M 1  is drawn upward in the vertical direction (i.e., toward the z-axis direction plus side). Next, at time t2 to t3, the molten metal M 1  is drawn up inclined toward the x-axis direction plus side with respect to up direction in the vertical direction. At this time, the solidification interface on the side captured by the imaging portion  109  is lower than the solidification interface at time t1 to t2. Lastly, at time t3 to t4, the molten metal M 1  is drawn up inclined toward the x-axis direction minus side with respect to up direction in the vertical direction. At this time, the solidification interface on the side captured by the imaging portion  109  is higher than the solidification interface at time t1 to t2. 
       FIG. 12  is a view of the relationship between the interface detection rate and the position of the solidification interface (i.e., a view of the test results). As shown in  FIG. 12 , the interface detection rate is extremely low, at 30% or 0%, without the pattern P when the interface position is medium or low. This is because it is difficult to identify the solidification interface without the pattern P when the interface position is relatively low. In contrast, with the pattern P, the interface detection rate is approximately 100% regardless of the interface position (i.e., even when the interface position is low). This is because it is possible to identify the solidification interface, regardless of the interface position, when the pattern P is provided. 
     Second Example Embodiment 
     Next, a free casting apparatus according to a second example embodiment of the invention will be described with reference to  FIGS. 13 and 14 .  FIG. 13  is a plan view of a shape determining member  202  according to the second example embodiment.  FIG. 14  is a side view of the shape determining member  202  according to the second example embodiment. The xyz coordinates in  FIGS. 13 and 14  also match those in  FIG. 1 . 
     The shape determining member  102  according to the first example embodiment shown in  FIG. 2  is formed from one plate, so the thickness t1 and width w1 of the molten metal passage portion  103  are fixed. In contrast, the shape determining member  202  according to the second example embodiment includes four rectangular shape determining plates  202   a,    202   b,    202   c,  and  202   d,  as shown in  FIG. 13 . That is, the shape determining member  202  according to the second example embodiment is divided into a plurality of sections. This kind of structure enables the thickness t1 and width w1 of the molten metal passage portion  203  to be changed. Also, the four rectangular shape determining plates  202   a,    202   b,    202   c,  and  202   d  are able to be synchronously moved in the z-axis direction. Moreover, the pattern P is applied to the upper surface of the shape determining member  202 , similar to the shape determining member  102 . 
     As shown in  FIG. 13 , the shape determining plates  202   a  and  202   b  are arranged facing each other lined up in the y-axis direction. Also, as shown in  FIG. 14 , the shape determining plates  202   a  and  202   b  are arranged at the same height in the z-axis direction. The distance between the shape determining plates  202   a  and  202   b  determines the width w1 of the molten metal passage portion  203 . The shape determining plates  202   a  and  202   b  are able to move independently in the y-axis direction, so they are able to change the width w1.A laser displacement gauge S 1  may be provided on the shape determining plate  202   a,  and a laser reflecting plate S 2  may be provided on the shape determining plate  202   b,  as shown in  FIGS. 13 and 14 , in order to measure the width w1 of the molten metal passage portion  203 . 
     Also, as shown in  FIG. 13 , the shape determining plates  202   c  and  202   d  are arranged facing each other lined up in the x-axis direction. Also, the shape determining plates  202   c  and  202   d  are arranged at the same height in the z-axis direction. The distance between the shape determining plates  202   c  and  202   d  determines the thickness t1 of the molten metal passage portion  203 . Also, the shape determining plates  202   c  and  202   d  are able to move independently in the x-axis direction, so they are able to change the thickness t1. The shape determining plates  202   a  and  202   b  are arranged contacting upper sides of the shape determining plates  202   c  and  202   d.    
     Next, the drive mechanism of the shape determining plate  202   a  will be described with reference to  FIGS. 13 and 14 . As shown in  FIGS. 13 and 14 , the drive mechanism of the shape determining plate  202   a  includes slide tables T 1  and T 2 , linear guides G 11 , G 12 , G 21 , and G 22 , actuators A 1  and A 2 , and rods R 1  and R 2 . The shape determining plates  202   b,    202   c,  and  202   d  also each include a drive mechanism, similar to the shape determining plate  202   a,  but these are not shown in  FIGS. 13 and 14 . 
     As shown in  FIGS. 13 and 14 , the shape determining plate  202   a  is placed on and fixed to the slide table T 1  that is able to slide in the y-axis direction. The slide table T 1  is slidably placed on the pair of linear guides G 11  and G 12  that extend parallel to the y-axis direction. Also, the slide table T 1  is connected to the rod R 1  that extends in the y-axis direction from the actuator A 1 . This kind of structure enables the shape determining plate  202   a  to slide in the y-axis direction. 
     Also, as shown in  FIGS. 13 and 14 , the linear guides  11  and  12 , and the actuator A 1 , are placed on and fixed to the slide table T 2  that is able to slide in the z-axis direction. The slide table T 2  is slidably placed on the pair of linear guides G 21  and G 22  that extend parallel to the z-axis direction. Also, the slide table T 2  is connected to the rod R 2  that extends in the z-axis direction from the actuator A 2 . The linear guides G 21  and G 22 , and the actuator A 2 , are fixed to a horizontal floor or base, not shown, or the like. This kind of structure enables the shape determining plate  202   a  to slide in the z-axis direction. The actuators A 1  and A 2  may be hydraulic cylinders, air cylinders, or electric motors or the like, for example. 
     Next, a solidification interface control method according to the second example embodiment of the invention will be described with reference to  FIG. 15 .  FIG. 15  is a flowchart illustrating the solidification interface control method according to the second example embodiment. In  FIG. 15 , the steps up to step ST 4  are the same as those in the first example embodiment shown in  FIG. 5 , so a detailed description of these steps will be omitted. 
     If the position of the solidification interface is within the reference range (i.e., YES in step ST 3 ), the casting controlling portion  111  determines whether the dimensions (i.e., the thickness t and the width w) at the solidification interface determined by the image analyzing portion  110  are within the dimensional tolerance of the casting M 3  (step ST 11 ). Here, the dimensions (i.e., the thickness t and the width w) at the solidification interface are obtained simultaneously when the image analyzing portion  110  determines the solidification interface. If the dimensions obtained from the image are not within the dimensional tolerance (i.e., NO in step ST 11 ), the thickness t1 and the width w1 of the molten metal passage portion  103  are changed (step ST 12 ). Then the casting controlling portion  111  determines whether casting is complete (step ST 5 ). 
     If the dimensions are within the dimensional tolerance (i.e., YES in step ST 11 ), the process proceeds directly on to step ST 5  without changing the thickness t1 and the width t1 of the molten metal passage portion  103 . If casting is not complete (i.e., NO in step ST 5 ), the process returns to step ST 1 . On the other hand, if casting is complete (i.e., YES in step ST 5 ), control of the solidification interface ends. The other structure is the same as that in the first example embodiment, so a description thereof will be omitted. 
     In this way, with the free casting method according to the second example embodiment, the pattern P is applied to the upper surface of the shape determining member  202 , an image of the pattern P that is reflected onto an area near the solidification interface is captured, and the solidification interface is determined from this image, similar to the first example embodiment. Because the pattern P is reflected onto the retained molten metal M 2 , the brightness of the surface of the retained molten metal M 2  greatly changes when the retained molten metal M 2  oscillates slightly. Therefore, the solidification interface is able to be determined even when the solidification interface is low and oscillation of the molten metal is small. As a result, even if the solidification interface is low, feedback control for keeping the solidification interface within the predetermined reference range is able to be performed, so the dimensional accuracy and surface quality of the casting are able to be improved. 
     Furthermore, with the free casting method according to the second example embodiment, the thickness t1 and the width w1 of the molten metal passage portion  203  of the shape determining member  202  are able to be changed. Therefore, when determining the solidification interface from the image, the thickness t and the width w at the solidification interface are measured, and the thickness t1 and the width w1 of the molten metal passage portion  203  are changed if this measured value is not within the dimensional tolerance. That is, feedback control for keeping the dimensions of the casting within the dimensional tolerance is able to be performed. As a result, the dimensional accuracy of the casting is able to be improved even more. 
     The invention is not limited to the example embodiments described above, and may be modified as appropriate without departing from the spirit of the invention.