Method and device for observing crystal grains in molten metal

Abstract

To provide a method and device for observing crystal grains that crystallize and grow in a flowing molten metal using X-rays.SOLUTION: A method disclosed herein comprises: applying a rotating magnetic field to a crucible having a center axis and a roughly round tubular interior about the center axis from an outer periphery thereof using an electromagnetic coil, so as to cause a molten metal in the crucible to flow about the center axis; and irradiating an upper part or lower part of the crucible with X-rays in a direction parallel to the center axis to measure an intensity radio of the X-rays transmitting through the molten metal in a two-dimensional plane on the other side of the crucible using a two-dimensional detector.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for observing crystal grains in a molten metal, and more particularly to a method and an apparatus for observing, by using X-rays, crystal grains that crystallize and grow in a flowing molten metal. [Background technology]

[0002] In a method for observing the internal structure of a molten metal through which visible light cannot penetrate, the use of physical rays, particularly X-rays, is considered. This method makes it possible to visualize the local absorption of X-rays that can penetrate the molten metal, and enables in-situ observation of the solidification and segregation of the molten metal and the formation behavior of crystallized substances.

[0003] Non-Patent Document 1 describes a method for observing the nucleation and growth behavior of dross in a molten Zn bath using X-ray transmission imaging. A 100 μm Zn alloy sample is sandwiched between two single crystal sapphire plates and placed on the X-ray path, and is heated to above its melting point to melt. Even when melted, the Zn alloy sample does not change position between the two single crystal sapphire plates due to viscosity and surface tension, and is held on the X-ray path. X-rays are irradiated from the upstream side, pass through the sample, and are absorbed and scattered to a certain extent before reaching the image detector, where the difference in the X-ray absorption coefficient in the sample is detected and recorded as an image with two-dimensional contrast differences as a change in X-ray intensity.

[0004] Patent Document 1 also discloses a method for visualizing the behavior of molten metal supplied inside a mold during casting using X-rays. A test mold having a predetermined thickness through which X-rays can penetrate is used, and X-rays are irradiated from one side of the mold, and the intensity of the X-rays that pass through and emerge from the other side is converted into the intensity of visible light to visualize the behavior using an imaging tube that forms a transmitted image. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-162568 A [Non-patent literature]

[0006] [Non-Patent Document 1] In-situ observation of crystallization and growth behavior of Fe2Al5 intermetallic compound in molten Zn bath using transmission X-ray imaging method; Shosei Katsura, Ryo Sasaki, Noriaki Nakatsuka, Hideyuki Yasuda; Tetsu-to-Hagané, Vol. 105, No. 7 (2019) Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, the use of X-rays makes it possible to observe the internal structure of molten metal. Here, the crystal grains that crystallize and grow in a flowing molten metal grow under the influence of the flow state, and there was a need to perform observations using X-rays while controlling this flow state.

[0008] The present invention has been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a method and an apparatus for observing, by using X-rays, crystal grains that crystallize and grow in a flowing molten metal. [Means for solving the problem]

[0009] The observation method according to the present invention is a method for observing crystal grains that crystallize and grow in a flowing molten metal, and is characterized in that a rotating magnetic field is applied around the central axis from the outer periphery of a crucible having an approximately circular tubular inner surface and a central axis, the molten metal contained in the crucible is caused to flow around the central axis, X-rays are irradiated from the top or bottom of the crucible parallel to the central axis, and the intensity ratio in a two-dimensional plane of the X-rays that penetrate the molten metal on the opposite side of the crucible is measured.

[0010] According to this feature, the flow state of the molten metal can be controlled by the rotating magnetic field, and X-rays can be irradiated and transmitted therethrough, making it possible to observe crystal grains that crystallize and grow in the flowing molten metal.

[0011] In the above-mentioned invention, the molten metal may be solidified while flowing. According to this feature, it is possible to observe crystal grains that crystallize and grow while the molten metal is cooled while flowing.

[0012] In the above-mentioned invention, the intensity ratio may be integrated for a predetermined time to obtain a two-dimensional image. According to this feature, a two-dimensional image having sufficient contrast can be obtained, and crystal grains that crystallize and grow in a flowing molten metal can be observed using X-rays.

[0013] In addition, the observation device according to the present invention is an observation device for observing crystal grains that crystallize and grow in a flowing molten metal, and is characterized by including: a stage for placing a crucible having an approximately circular tubular inner surface and a central axis; an electromagnetic coil arranged around the stage and applying a rotating magnetic field around the central axis from the outer periphery of the crucible, thereby causing the molten metal contained in the crucible to flow around the central axis; an X-ray source arranged above or below the stage and irradiating the molten metal in the crucible with X-rays parallel to the central axis; and a two-dimensional detector arranged opposite the X-ray source across the stage and measuring the intensity ratio in a two-dimensional plane of the X-rays that pass through the molten metal on the opposite side of the crucible.

[0014] According to this feature, the flow state of the molten metal can be controlled by the rotating magnetic field generated by the electromagnetic coil, and X-rays from the X-ray source can be irradiated to the molten metal and transmitted to the two-dimensional detector, so that crystal grains that crystallize and grow in the flowing molten metal can be observed using X-rays.

[0015] In the above-mentioned invention, a control unit may be included that controls an input to the electromagnetic coil and controls a flow rate of the molten metal in the crucible. According to this feature, it is possible to observe crystal grains that crystallize and grow in the flowing molten metal while controlling the flow state of the molten metal with the control unit.

[0016] In the above-mentioned invention, the present invention may be characterized in that it includes a temperature control unit that controls the temperature of the molten metal. According to this feature, it is possible to observe, by using X-rays, crystal grains that crystallize and grow in the flowing molten metal while controlling the solidification of the molten metal.

[0017] In the above-mentioned invention, the two-dimensional detector may be characterized in that it integrates the intensity ratio for a predetermined time to provide a two-dimensional image. With this feature, a two-dimensional image with sufficient contrast can be obtained, and crystal grains that crystallize and grow in a flowing molten metal can be observed using X-rays. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram of an observation device for crystal grains in a molten metal. [Diagram 2] In the observation example, (a) is a structural photograph of a vertical cross section of a solidified body when no flow is applied, and (b) is a structural photograph of a solidified body when flow is applied. [Diagram 3] The observation examples show X-ray images of molten metal that has been given fluidity during cooling at (a) 720.6°C, (b) 684.9°C, (c) 671.6°C, and (d) 600.4°C. [Figure 4] The X-ray images of the molten metal that was not given fluidity in the observation example during natural cooling at (a) 719.4°C, (b) 683.6°C, (c) 670.9°C, and (d) 601.0°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] An observation device for observing crystal grains in a molten metal and a method for observing crystal grains using the observation device according to one embodiment of the present invention will be described below with reference to Fig. 1. The contrast of an X-ray transmission image according to the present invention is obtained by the difference in density and / or composition of particles precipitated in the molten metal from those of the molten metal itself.

[0020] As shown in FIG. 1, the observation device 10 is a device for visualizing and observing the change over time in the growth morphology of crystal grains crystallized under the flow of molten metal, and includes a crucible 1 for containing and holding the molten metal, and an electromagnetic coil 2 for horizontally rotating the molten metal in the crucible 1 to cause it to flow. The observation device 10 also includes an X-ray source 3 for irradiating the molten metal with X-rays, and a two-dimensional detector 4 for measuring the intensity ratio in a two-dimensional plane of the X-rays penetrating the molten metal. It is also preferable to include a control unit 5 for controlling the input to the electromagnetic coil 2 and controlling the flow speed of the molten metal in the crucible 1. For example, a personal computer can be used as the control unit 5. The control unit 5 is also connected to the X-ray source 3 and the two-dimensional detector 4, and can control the operations of these as well.

[0021] The crucible 1 is a bottomed cylindrical body whose inner surface is made substantially cylindrical to ensure a smooth flow of the molten metal, and is placed at the center of a stage 12 that extends substantially horizontally inside a housing 11 of the observation device 10. The crucible 1 is placed with the central axis L of its cylindrical inner surface facing substantially vertically. As will be described later, X-rays are irradiated parallel to the central axis L.

[0022] Stage 12 has a hole 12a in the center so as not to obstruct the path of the X-rays that have passed through the molten metal. A stand 6 that supports crucible 1 is placed on top of hole 12a. Stand 6 is also configured to support, for example, the outer periphery of crucible 1 so as not to obstruct the path of the X-rays that have passed through the molten metal.

[0023] The X-ray source 3 and the two-dimensional detector 4 are disposed on opposite sides of the crucible 1 with the stage 12 in between. That is, the X-ray source 3 and the two-dimensional detector 4 are disposed on the upper or lower part of the stage 12, respectively. Here, an example in which the X-ray source 3 is disposed on the upper part of the stage 12 is shown. The X-ray source 3 is configured to irradiate X-rays downward so as to be parallel to the central axis L. The two-dimensional detector 4 is disposed further below the bottom plate 13 of the housing 11, and a window (not shown) is provided in the center of the bottom plate 13 so as to expose the two-dimensional detector 4 to the X-rays it receives. Casters 8 are provided on the lower surface of the bottom plate 13 for moving the observation device 10.

[0024] As described above, the inner surface of the crucible 1 is substantially cylindrical, and X-rays are irradiated parallel to the central axis L. This limits the obstacle on the path of the X-rays passing through the molten metal to the bottom of the crucible 1, suppressing attenuation of the X-rays. The X-rays emitted from the X-ray source 3 installed above the crucible 1 enter and pass through the molten metal in the crucible 1, and then enter the two-dimensional detector 4 installed below the crucible 1. A transmission image of the molten metal is obtained from the X-ray intensity ratio in a two-dimensional plane measured by the two-dimensional detector 4.

[0025] Here, the ratio of the shortest distance from the X-ray source 3 to the two-dimensional detector 4 (FID: Focus to Image receptor Distance) to the shortest distance from the X-ray source 3 to the molten metal in the crucible 1 (FOD: Focus to Object Distance) (ratio of FID to FOD) is, for example, 100 or less, and preferably 2 to 10. If the ratio of FID to FOD is made small, the distance between the electromagnetic coil 2 arranged around the crucible 1 and the two-dimensional detector 4 may be too close, causing the two-dimensional detector 4 to be affected by the electromagnetic field and reducing the S / N ratio of the captured image. Also, if the ratio of FID to FOD is made large, the X-ray detection efficiency may be reduced, reducing the contrast of the transmitted image.

[0026] The X-ray source 3 includes an X-ray tube in which a target is enclosed and a power supply device that supplies a direct current to the X-ray tube, and the values ​​of the acceleration voltage and tube current can be set by the control unit 5. The X-ray source 3 is slidably attached to a slide rail (not shown) provided on the ceiling of a housing 11 made of an aluminum frame, for example, and is movable in the horizontal direction. When the crucible 1 is inserted or removed from an insertion port (not shown) provided in the upper part of the housing 11, the X-ray source 3 can be moved aside, making it easy to insert or remove the crucible 1.

[0027] The tube voltage of the X-ray source 3 is preferably 80 kV or more, more preferably 100 kV to 150 kV. The tube current is preferably 100 μA or more, more preferably 300 μA or more. If the tube voltage and tube current are small, the X-ray intensity per unit time is low, making it difficult to obtain a clear transmission image of the molten metal. On the other hand, if the tube current is increased, the X-ray intensity per unit time is increased, and the detection sensitivity is improved. However, even if the tube voltage and tube current satisfy the above conditions, if the focal spot size is larger than the effective resolution of the detector, the greater the distance between the molten metal and the two-dimensional detector 4, the more blurred the transmission image obtained, making it unsuitable for observing crystals. To observe crystals of 20 μm or less, it is necessary to use an X-ray source with a focal spot size of 10 μm or less. If the tube voltage is too large, the contrast of the X-ray image is reduced.

[0028] The two-dimensional detector 4 is equipped with a phosphor, a photodiode, and a thin-film transistor, and measures the intensity ratio of the incident X-rays and converts them into an electrical signal for output. By connecting to the control unit 5, it is possible to set the integration time for imaging, and it is possible to perform imaging of the transmitted image during the solidification process of the molten metal from the intensity of the transmitted X-rays.

[0029] The electromagnetic coil 2 applies a rotating magnetic field around the central axis L to the molten metal in the crucible 1 from the outer periphery of the crucible 1. The molten metal to which the rotating magnetic field is applied is induced to flow by the rotating magnetic field, and a flow occurs around the central axis L so as to rotate along the inner peripheral surface of the crucible 1.

[0030] The electromagnetic coil 2 that applies such a rotating magnetic field can be, for example, the same as the stator coil of a three-phase AC motor. That is, three stator coils are arranged around the stage 12 at equal intervals (120° intervals in the circumferential direction) centered on the central axis L. Then, by passing a three-phase AC current through the three stator coils, a rotating magnetic field can be applied in the same manner as a motor. In this way, by arranging the electromagnetic coil 2 around the outer periphery of the crucible 1, it is possible to suppress attenuation of the X-rays without becoming an obstacle on the path of the X-rays irradiated to the molten metal in the crucible 1. It is preferable to control the application of an AC current to the stator coil via an inverter 7, since the rotation speed and rotation direction of the rotating magnetic field can be controlled by adjusting the frequency of the AC current.

[0031] The metal sample contained in the crucible 1 is melted in advance in an electric furnace. Any electric furnace can be used as long as it can accommodate the crucible and melt the metal sample. For example, if the sample is an aluminum alloy, a known muffle furnace can be used.

[0032] The material of the crucible is preferably one with low X-ray absorption, and examples of such materials include carbon, boron nitride, and alumina. The material and dimensions of the crucible 1 affect the cooling rate of the molten metal. The cooling rate of the molten metal is, for example, 10°C / s or less, preferably 5°C / s or less, and more preferably 2°C / s or less. If the cooling rate of the molten metal is high, the crystals of the separation object that crystallizes will grow too fast, which increases the blur of the subject of the transmission image and makes it difficult to obtain a high-contrast transmission image. Therefore, it is also preferable to include a temperature control unit that controls the temperature of the molten metal using an electric heating wire or the like that heats the crucible 1. The temperature control unit can control the cooling rate of the molten metal and adjust the growth rate of the crystals that crystallize.

[0033] Furthermore, when capturing a transmission image, it is preferable to obtain a two-dimensional image by integrating the intensity ratio of the X-rays measured by the two-dimensional detector 4 for a predetermined time. The predetermined time is preferably short from the viewpoint of reducing blurring of the transmission image caused by the flow of the molten metal, and is preferably long from the viewpoint of increasing the contrast of the obtained transmission image. The integration time is determined taking these factors into consideration.

[0034] The cylindrical crucible 1 preferably has an inner diameter of 10 mm to 500 mm, more preferably 30 mm to 100 mm. If the inner diameter is too small, it is expected to be difficult to induce flow in the molten metal by the rotating magnetic field. Also, if the inner diameter is too large, it is difficult to select an X-ray source and X-ray detector that can transmit and photograph the entire molten metal, which is not practical.

[0035] According to the observation device 10 as described above, the flow state of the molten metal can be controlled by the rotating magnetic field, and crystal grains that crystallize and grow in the flowing molten metal can be observed using X-rays. For example, the molten metal is allowed to flow and allowed to cool and solidify. Then, the molten metal crystallizes and grows crystal grains while flowing. The crystallization and growth of such crystal grains can be observed with the observation device 10. From the viewpoint of flowing the molten metal with a magnetic field, it may be difficult to obtain sufficient flow of the molten metal in an alloy containing a semiconductor element with low electrical conductivity, for example, silicon or germanium, as a first element.

[0036] By applying a rotational flow to the molten metal, crystal grains crystallizing in the molten metal can be separated as impurities by centrifugation, and the metal can be refined. In this case, the molten metal contains the metal to be refined and the objects to be separated, which are impurities. Since such objects to be separated are separated from the molten metal by centrifugation, they have a higher melting point than the metal to be refined, and are non-metallic inclusions such as oxides and sulfides, nitrides, carbides, intermetallic compounds, etc. that crystallize as crystals in the liquid phase of the molten metal during cooling. In other words, the crystallization and growth of such impurities can be observed by the observation device 10.

[0037] Assuming the above-mentioned centrifugation, the rotation speed of the rotating magnetic field is preferably 1 s -1 (hereinafter referred to as rps) or more and 100 rps or less, more preferably 20 rps or more and 60 rps or less. If the rotation speed of the rotating magnetic field is too small, the rotation speed due to the flow of the molten metal is also small, and there is a possibility that the object to be separated cannot be sufficiently separated because of insufficient centrifugal force. On the other hand, if the rotation speed of the rotating magnetic field is too large, the centrifugal force is strong and the surface of the molten metal at the wall surface of the crucible 1 is significantly raised, so that the molten metal may not be held in the crucible 1. From the viewpoint of stably holding the molten metal in the crucible 1 while separating the object to be separated well, it is preferable that the rotation speed of the rotating magnetic field is set within the above range. However, the rotation speed of the rotating magnetic field is not limited to the above examples, and may be changed appropriately depending on the type and concentration of the object to be separated and the diameter of the crucible.

[0038] It is also preferable to reverse the rotation caused by the flow of the molten metal by reversing the rotating magnetic field. In the case of unidirectional rotation, the surface of the molten metal rises on the crucible wall surface due to the centrifugal force of the molten metal, resulting in a difference in the transmission distance of the X-ray between the center and the outer periphery, resulting in unintended contrast in the transmission image. On the other hand, by applying reverse rotation, the rise of the surface of the molten metal on the crucible wall surface can be suppressed. In this case, the reversal period is preferably in the range of 0.1 s to 5.0 s, more preferably in the range of 0.2 s to 2.0 s. If the reversal period is too short, the direction of rotation of the magnetic field is reversed before sufficient strength of the rotating magnetic field is obtained, making it difficult to induce sufficient flow in the molten metal. If the reversal period is too long, the effect of reversal is reduced, and for example, it becomes impossible to suppress the rise of the surface of the molten metal on the crucible wall surface.

[0039] [Observation example] The results of observing crystal grains that crystallize and grow in a molten metal using the observation device 10 will be described.

[0040] An Al-10%Si-2%Fe-2%Mn alloy was used as the metal for the molten metal. The electromagnetic coil 2 was a stator coil (bore diameter: 100 mm, output: 2.2 kW) of a three-phase, four-pole AC motor. The X-ray source 3 was a microfocus X-ray source G-311 (tube voltage: 110 kV, tube current: 272 μA, focal spot size: 8 μm) manufactured by Canon Anelva Corporation. The two-dimensional detector 4 had a pixel size of 83 μm. The FOD and FID were 140 mm and 800 mm, respectively, and the effective resolution was 15 μm. The integration time for capturing the transmitted image was 1 second (1 image / s). The temperature of the molten metal during cooling was measured by inserting a K thermocouple from above into the center of the molten metal.

[0041] About 50 g of the above alloy was placed in a graphite crucible with an inner diameter of 50 mm, and the alloy was heated to 850°C in an electric furnace to melt the alloy and obtain a molten metal. The crucible was removed from the electric furnace, and the dross on the surface of the molten metal was removed, and then the crucible was placed on the observation device 10. The molten metal was solidified by natural cooling while a rotating magnetic field was applied by an electromagnetic coil to cause the molten metal to flow. The rotating magnetic field was continuously applied from the time the crucible was placed in the observation device 10 until the molten metal was completely solidified. A voltage with a frequency of 120 Hz was input to the electromagnetic coil so that the rotation speed of the rotating magnetic field was 60 rps. In addition, the rotating magnetic field was rotated in both directions with a reversal period of 1.0 s to cause the molten metal to flow. The state of the crystal grains during the solidification of the molten metal was observed, and after the cooling was completed, the solidified body of the molten metal was cut vertically into two equal parts along the central axis L, and the mirror polished structure of the cross section was also observed. For comparison, the same observation was also made when no rotating magnetic field was applied.

[0042] FIG. 2 shows the mirror polished micrographs of the vertical cross section of each solidified body when (a) a rotating magnetic field was not applied and (b) a rotating magnetic field was applied (120 Hz, 1 s reverse rotation). The bright structure is α-aluminum, and the low-brightness polygonal structure is the intermetallic compound α-AlSiFeMn. As shown in FIG. 2(a), when a rotating magnetic field was not applied, α-AlSiFeMn was distributed over the entire cross section and was present in the inner peripheral part as well as the outer peripheral part. A eutectic structure of α-aluminum and silicon was formed between the α-AlSiFeMn. In contrast, as shown in FIG. 2(b), when a rotating magnetic field was applied, the proportion of α-AlSiFeMn in the center was lower than when a rotating magnetic field was not applied, and it was confirmed that α-AlSiFeMn was unevenly distributed in the outer peripheral part.

[0043] FIG. 3 shows the transmission images of the molten metal during cooling when a rotating magnetic field is applied. The transmission images were taken at the following temperatures during cooling: (a) 720.6°C, (b) 684.9°C, (c) 671.6°C, and (d) 600.4°C. As shown in FIG. 3(a), the brightness of the image corresponding to the molten metal was almost constant at 720.6°C. As shown in FIG. 3(b), the formation of a low-brightness region was observed on the outer periphery at 684.9°C. As shown in FIG. 3(c), this low-brightness region expanded its range at 671.6°C, and as shown in FIG. 3(d), it extended over the entire outer periphery at 600.4°C. In addition, at the same temperature, many high-brightness annular regions were formed in the center of the image corresponding to the molten metal. From the observation results of the mirror-polished structure of the cross section of the solidified body described above, it was found that the low-brightness structure corresponds to an intermetallic compound containing Fe and Mn, and the high-brightness structure corresponds to a porosity.

[0044] Figure 4 shows the transmission images of the molten metal during cooling without applying a rotating magnetic field. The transmission images were taken at temperatures of (a) 719.4°C, (b) 683.6°C, (c) 670.9°C, and (d) 601.0°C during cooling. As shown in (a) of the figure, the brightness of the image corresponding to the molten metal was almost constant at 719.4°C. As shown in (b) of the figure, a ring-shaped area with high brightness was observed throughout the image corresponding to the molten metal at 683.6°C, which is considered to be the formation of a cavities in the molten metal. As shown in (c) of the figure, the number and size of the ring-shaped areas increased with decreasing temperature (670.9°C). As shown in (d) of the figure, although the formation of some low brightness areas was observed in the outer periphery at 601.0°C, compared to the case of applying a rotating magnetic field (Figure 3), the low brightness areas in the outer periphery were small, and the formation of low brightness areas was also observed in the center. In other words, it is believed that in the solidified body obtained without fluidizing the molten metal, intermetallic compounds containing Fe and Mn were formed throughout.

[0045] As described above, the observation device 10 was able to use X-rays to observe crystal grains that crystallize and grow in a flowing molten metal. It was also found that by solidifying the molten metal while giving it a flow, the region in which the intermetallic compound is formed is biased toward the outer periphery.

[0046] In addition to the aluminum scrap alloys listed in the examples, other examples of the observation objects include aluminum-silicon hypereutectic alloys and aluminum-silicon-copper alloys. In such alloys, the present invention can be applied to the visualization of the crystal growth of primary silicon and eutectic crystals formed during solidification. In addition, since the molten alloy is given a fluidity by electromagnetic stirring, the present invention can be applied to the observation of alloys containing metal elements such as magnesium, iron, copper, zinc, and tin as the first element, in addition to aluminum. Therefore, it is possible to perform solidification imaging of molten scrap alloys containing these metals as the main components, and to visualize the crystal growth of intermetallic compounds crystallized in a metal plating bath. [Industrial Applicability]

[0047] The observation method and apparatus according to the present invention can visualize each process of the inclusion of nonmetallic inclusions, which are problematic in manufacturing in the field of casting aluminum alloys, and the formation of intermetallic compounds containing impurity elements such as iron, manganese, and copper that crystallize in molten aluminum scrap, and as a result, can bring about improvements in the manufacturing process. [Explanation of symbols]

[0048] 1 Crucible 2. Electromagnetic coil 3 X-ray source 4. Two-dimensional detector 10 Observation equipment L center axis

Claims

1. A method for observing crystal grains that crystallize and grow in a flowing molten metal, comprising the steps of: A rotating magnetic field is applied from an outer periphery of a crucible having a central axis and an inner surface of a substantially circular tube, around the central axis, to cause the molten metal contained in the crucible to flow around the central axis; A method for observing crystal grains in a molten metal, comprising: irradiating X-rays from the top or bottom of the crucible parallel to the central axis; and measuring the intensity ratio in a two-dimensional plane of the X-rays that penetrate the molten metal on the opposite side of the crucible.

2. 2. A method for observing crystal grains in a molten metal according to claim 1, wherein the molten metal is allowed to flow while being cooled, and observation of crystal grains which crystallize and grow is performed.

3. 3. The method for observing crystal grains in a molten metal according to claim 1, wherein the intensity ratio is integrated for a predetermined time to obtain a two-dimensional image.

4. An apparatus for observing crystal grains that crystallize and grow in a flowing molten metal, comprising: a stage for disposing a crucible having a central axis and an inner surface of a substantially circular tube; an electromagnetic coil disposed around the stage and configured to apply a rotating magnetic field around the central axis from the outer periphery of the crucible and cause the molten metal contained in the crucible to flow around the central axis; an X-ray source disposed above or below the stage and configured to irradiate the molten metal in the crucible with X-rays parallel to the central axis; and a two-dimensional detector that is disposed opposite the X-ray source across the stage and that measures the intensity ratio in a two-dimensional plane of the X-rays that pass through the molten metal on the opposite side of the crucible.

5. 5. The apparatus for observing crystal grains in a molten metal according to claim 4, further comprising a control unit for controlling an input to the electromagnetic coil and controlling a flow rate of the molten metal in the crucible.

6. 6. The apparatus for observing crystal grains in a molten metal according to claim 4, further comprising a temperature control unit for controlling a temperature of the molten metal.

7. 5. An apparatus for observing crystal grains in a molten metal according to claim 4, wherein said two-dimensional detector integrates said intensity ratio for a predetermined time period to provide a two-dimensional image.

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