JP2016107312A - Method for generating air bubble in molten metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method by which a cost can be suppressed and fine air bubbles can be generated in molten metal.SOLUTION: A method for generating air bubbles in molten metal by blowing inert gas into the molten metal is provided that comprises: a step in which an electric potential difference is given between a nozzle which has a gas blowing hole and is made of a conductive substance and the molten metal to thereby electrically conduct the molten metal, and thus, a contact angle of the molten metal with respect to the nozzle is controlled to be less than 90°; and a step in which the inert gas is blown into the molten metal from the gas blowing hole while the contact angle is controlled to be less than 90°.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for generating fine bubbles in molten metal by gas injection.

  In a dry refining process that handles molten metal, impurity elements such as carbon and nitrogen in the molten metal, or non-metallic inclusions mainly composed of oxides may adversely affect the characteristics of products manufactured from the molten metal. Many. Therefore, it is important to remove these impurity elements and non-metallic inclusions. In particular, if non-metallic inclusions remain in the product obtained from the molten metal, which leads to a serious defect of the product, it must be removed in the molten state, that is, before the casting.

  As a means for removing non-metallic inclusions in the molten metal, it is effective to generate inert gas bubbles from the nozzle by inserting the nozzle into the inert gas, that is, the molten metal. By adhering the bubbles to the non-metallic inclusions, the non-metallic inclusions float on the surface of the molten metal and separate from the molten metal, so that the cleanliness of the molten metal is improved.

  Improvement of cleanliness of molten metal using gas injection is largely related to the form of bubbles generated. That is, the smaller the bubble size or the greater the number of bubbles, the greater the probability of adhering to the nonmetallic inclusions and floating the nonmetallic inclusions.

  When gas injection is performed on water at normal temperature and normal pressure instead of molten metal, it is easy to generate bubbles having a small size. However, even when nozzles having the same inner diameter and outer diameter are used, there is a limit in generating small-sized bubbles that can be obtained in an aqueous system when injected into molten metal. This is considered to be related to the fact that the surface tension of the molten metal is larger than the surface tension of water.

  Patent Document 1 discloses a method in which gas is blown through a porous brick provided at the bottom of a container for containing molten steel to generate bubbles in the molten steel. In this method, a fluctuating electromagnetic force is generated in the molten steel in the container, the apparent pressure acting on the surface of the porous body is fluctuated, and the diameter of bubbles rising from the surface of the porous body is controlled. In Patent Document 1, as a method of generating an electromagnetic force that fluctuates in the vertical direction in the molten steel, it is mentioned that a static magnetic field and an alternating current that are orthogonal to each other are applied to the molten steel in the horizontal direction.

  In Patent Document 2, in a method of generating bubbles by blowing a gas into molten metal, a direct current is applied to the molten metal in a portion where bubbles are generated in the direction of generation of bubbles and generated by electromagnetic force generated thereby. A method for controlling the diameter of bubbles to be released is disclosed.

  In Patent Document 3, the entire circumferential wall of a cylindrical container in which molten steel is contained is a gas blowing zone for blowing inert gas, and a driving device for rotating the molten steel in a horizontal plane is disposed below the gas blowing zone. An apparatus is disclosed. Patent Document 3 describes that a velocity gradient is generated in molten steel by a driving device to disperse fine bubbles throughout the container.

Japanese Patent Publication No. 03-30456 Japanese Patent Laid-Open No. 06-128660 JP 09-122846 A

  However, in the method of Patent Document 1, the equipment cost of a device for generating a static magnetic field is high, and further, the equipment size is unavoidable as the amount of molten steel in the container increases.

  In the methods of cited references 1 and 2, a method for obtaining conditions for generating fine bubbles is not disclosed, and the bubbles may not be refined in some cases.

  In the technique of Cited Document 3, at least the entire circumferential wall of the container containing the molten steel is made of a refractory, and the refractory is melted by the rotation of the molten steel. Is necessary and the operating cost increases.

  This invention is made | formed in view of said problem, and it aims at providing the method which suppresses cost and makes it possible to generate a fine bubble in a molten metal.

The gist of the present invention is the following method for generating bubbles in molten metal.
A method of generating bubbles in the molten metal by blowing an inert gas into the molten metal,
The contact angle of the molten metal with respect to the nozzle is controlled to less than 90 ° by applying a potential difference between the nozzle made of a conductive material having a gas blowing hole and the molten metal and energizing the molten metal. And a process of
A method for generating bubbles in the molten metal, the method comprising: blowing an inert gas from the gas blowing hole into the molten metal in a state where the contact angle is controlled to be less than 90 °.

  By this bubble generation method, fine bubbles can be generated in the molten metal. The potential difference between the molten metal and the nozzle can be applied, for example, by immersing a conductive member serving as a counter electrode in the molten metal and applying a potential difference between the counter electrode and the nozzle. Since the nozzle and the counter electrode can be manufactured at low cost, the present invention can suppress the cost.

FIG. 1 is a diagram showing an example of an apparatus configuration that can be used to implement the bubble generation method of the present invention.

[Reason why it is difficult to generate fine bubbles]
In general, the size of bubbles generated in a liquid by blowing a gas from a nozzle immersed in the liquid is determined by the nozzle diameter, the surface tension and density of the liquid, and the contact angle (wettability) between the nozzle and the liquid. Strongly affected. If the type of liquid is the same, it is effective to reduce the nozzle diameter or the contact angle between the nozzle and the liquid in order to reduce the size of the generated bubbles.

  However, when the contact angle between the nozzle and the liquid exceeds 90 °, that is, when the nozzle does not get wet with the liquid, no matter how small the nozzle diameter can be, there is a limit to the bubble miniaturization. This is because the gas that comes out of the nozzle outlet and grows as a bubble does not leave the nozzle until it grows to a critical bubble diameter. Therefore, in order to make the bubbles fine, it is necessary to reduce the diameter of the nozzle and simultaneously improve the wettability between the nozzle and the liquid.

  It is easy to reduce the nozzle diameter by making full use of various machining techniques such as electric discharge machining. On the other hand, no effective measures have been taken so far to improve the wettability between the nozzle and the liquid.

  If the liquid into which the gas is blown is a liquid at room temperature (for example, the liquid is water) and bubbles are generated at room temperature, apply a hydrophilic coating agent to the surface of the solid contacted by this liquid. Thus, the wettability between water and the solid can be easily increased. However, there is no coating agent that is stable and practical even at high temperatures (for example, several hundred degrees C. or more). Therefore, in the case of a liquid having a melting point of at least several hundred degrees Celsius, such as a molten metal, it is difficult to arbitrarily control the wettability between the nozzle and the liquid, and it can withstand use at high temperatures. The nozzle material that can be produced is also limited.

  Even if there is a material that can improve the wettability with the molten metal, such a material tends to react with the molten metal and be consumed. Therefore, if we focus on improving wettability and select a nozzle made of a consumable material that is easy to react, not only will there be a problem in durability, but the nozzle diameter will gradually increase as wear progresses. Therefore, the reduction in nozzle diameter and good wettability are not compatible.

[Concept of which the present invention was based]
In the case of gas injection into molten metal, the present inventors solved the above-mentioned durability problem by providing an appropriate potential difference between the nozzle and the molten metal. It has been found that the wettability between the two can be improved and fine bubbles can be generated in the molten metal. This finding was obtained from experiments that evaluated the wettability between molten metal and non-metallic materials by changing the conditions by trial and error. Hereinafter, this experiment will be described.

<Experiment 1> Evaluation of Wettability Between Molten Metal and Non-Metallic Material Using a chamber-type electric furnace, the inside of the furnace is an Ar atmosphere, and the metal is a disk-shaped disk having a diameter of 30 mm and a thickness of 10 mm It was melted on a substrate made of a non-metallic material.

The metals to be melted were the following metals A to C.
Metal A: Fe-4.5% by mass C (shape: cylindrical shape having a diameter of 8 mm and a thickness of 5 mm)
Metal B: Cu with a purity of 99.99% (shape: cylindrical shape with a diameter of 8 mm and a thickness of 5 mm)
Metal C: Fe with a purity of 99.99% (shape: cylindrical shape with a diameter of 7 mm and a thickness of 7 mm)

As the substrate, a substrate A made of graphite, a substrate B made of ZrO ₂ , and a substrate C made of Y ₂ O ₃ were used. When using metal A or metal B, the melting temperature of the metal was 1200 ° C., and the metal was melted in all combinations of metal A and metal B with substrate A, substrate B, and substrate C. On the other hand, when using metal C, the melting temperature of the metal was 1550 ° C., and the metal was melted only on substrate B and substrate C.

  In any case, the region including the contact portion between the molten metal and the substrate was photographed from the side with a CCD camera, and the contact angle of the molten metal with respect to the substrate was measured from the photographed image. The contact angles were all in the range of 120 to 150 °. That is, the contact angle was 90 ° or more, and the wettability of the molten metal to the substrate was low.

Next, the molten metal and the substrate were respectively connected to a direct current power supply device via a conductive member, and an electric circuit that allowed application of a potential difference between the molten metal and the substrate was configured. As a result, it was found that when a potential difference is applied between the molten metal and the substrate, the contact angle changes. In particular, when a potential difference is applied so that the current density is 35 mA / cm ^{2 to} 1 A / cm ² , the contact angle between the molten metal and the substrate is in the range of 46 to 90 ° regardless of the type of metal and substrate. I was able to control it. That is, a condition for improving wettability was found.

<Experiment 2> Gas Injection Test into Molten Metal Based on the knowledge obtained in Experiment 1, a gas injection test into the molten metal was carried out to find a method for generating fine bubbles in the molten metal.

First, among a pair of output terminals provided in the direct power supply device, a graphite rod is connected to the positive electrode, and a gas blowing tube (single tube) made of ZrO ₂ or graphite is connected to the negative electrode. . The diameter of the graphite rod was 3 mm. All of the gas blowing tubes had an outer diameter of 1 mm and an inner diameter of 0.4 mm. The tip of the gas blowing tube functions as a nozzle for discharging gas.

  Next, 3.5 kg of 99.9% purity Cu was charged into a bottomed cylindrical alumina crucible having an outer diameter of 96 mm, an inner diameter of 90 mm, and a height of 150 mm. After putting in the electric furnace and making the inside of the electric furnace an Ar atmosphere, copper was heated and melted and held at 1200 ° C.

Then, a graphite rod and a gas blowing tube were inserted into the Cu melt. The tip of the gas blowing tube was immersed to a depth of 45 mm from the surface of the Cu melt. In this state, a potential difference was applied between the graphite rod and the gas blowing tube by a DC power supply. Subsequently, Ar having a purity of 99.99% was blown into the Cu melt at a flow rate of 20 cm ³ / min (standard temperature and pressure; STP) via a gas blowing tube. As a result, the Ar gas discharged from the tip (nozzle) of the gas blowing tube floated up to the surface of the Cu melt as bubbles.

  At that time, depending on the conditions (material of the gas blowing tube tip, potential difference (voltage) applied by the DC power supply device), the surface of the Cu melt on which bubbles rise is observed from the top of the furnace by a camera. Taken as. Thereafter, the equivalent circle diameter of the bubbles was calculated from the obtained moving image by image analysis.

Table 1A shows the relationship between the applied voltage and the bubble diameter when a gas blowing tube having a tip made of ZrO ₂ is used. Table 1B shows the relationship between the applied voltage and the bubble diameter when a gas blowing tube having a graphite tip is used.

  Table 1A and Table 1B show the current density value at the surface of the gas blowing tube tip in the Cu melt, the contact angle of the Cu melt with respect to the material of the gas blowing tube tip, and the case where no voltage was applied. The ratio of the bubble diameter when a voltage is applied to the bubble diameter is also shown.

The value of the current density was obtained by dividing the output current by the contact area between the conduction part of the gas blowing tube (corresponding to a part made of graphite or ZrO ₂ ) and the Cu melt. The value of the contact angle is based on the result of Experiment 1.

From Table 1A and Table 1B, it can be seen that as the applied voltage increases and the current density in the Cu melt increases, the contact angle decreases and the bubble diameter decreases. Regardless of whether the material constituting the gas blowing tube tip is ZrO ₂ or graphite, when the contact angle of the Cu melt to the material is less than 90 °, it is 30% or more smaller than the bubble diameter when no voltage is applied. Diameter bubbles are generated.

  As described above, the bubble generation method of the present invention is a method for generating bubbles in a molten metal by blowing an inert gas into the molten metal, the nozzle having a gas blowing hole and made of a conductive material. And controlling the contact angle of the molten metal with respect to the nozzle to less than 90 ° by applying a potential difference between the molten metal and the molten metal, and reducing the contact angle to less than 90 ° And bubbling an inert gas from the gas blowing hole into the molten metal in a controlled state.

  When the contact angle is 90 ° or more, it is difficult for bubbles to separate from the nozzle, and it becomes difficult to generate fine bubbles. For this reason, a potential difference is applied between the molten metal and the nozzle so that the contact angle is less than 90 °. In order to accurately control the bubble diameter, the relationship between the potential difference and the contact angle is obtained in advance by a method according to Experiment 1 above, and when the bubble generation method of the present invention is performed, based on this relationship, It is preferable to apply a potential difference corresponding to the target contact angle between the molten metal and the nozzle.

By said energization, it is preferable to adjust the current density between said with the molten metal nozzle to ^{35mA / cm 2 ~1A / cm 2} . By setting the current density to 35 mA / cm ² or more, it is possible to generate fine bubbles with a contact angle of less than 90 °. On the other hand, when the current density exceeds 1 A / cm ² , the effect of generating fine bubbles is not only substantially saturated, but in order to generate such a current density, a large power supply device is required, Since it becomes necessary to thicken the wiring cable, it is not practically desirable. Accordingly, current density, and ^{35mA / cm 2 ~1A / cm 2} .

To get the effect stably, the current density is preferably controlled to ^{45mA / cm 2 ~900mA / cm 2} .

  In any of the first and second bubble generation methods of the present invention, the type of molten metal is not particularly limited. For example, Cu, Cu-based alloy, Fe, Fe-based alloy, Al, Al-based alloy, Ni, It can be made of a Ni-based alloy, stainless steel, or the like.

  In order to apply a potential difference between the nozzle and the molten metal, the nozzle (the edge of the hole through which the gas is blown) needs to be made of a conductive material. Examples of the conductive material constituting the nozzle include solid electrolytes such as zirconia and yttria, graphite, and graphite-based refractories such as alumina graphite and magnesia graphite.

  The nozzle can be, for example, the open end of a tube. When practicing the present invention, a single nozzle or a plurality of nozzles may be used. When the nozzle is used as the open end of the pipe, a single pipe or a plurality of pipes may be used. As the nozzle, a porous body (a member made of a porous material) may be used. In addition, the nozzle does not necessarily need to be immersed, and may be fitted into the bottom surface of a container containing molten metal, such as a ladle or tundish.

  According to the bubble generation method of the present invention, fine gas bubbles can be generated by performing gas injection on the molten metal. Since the non-metallic inclusions can be efficiently removed by the bubbles, it is possible to improve the refining efficiency and the cleanliness of the product obtained from the molten metal.

  FIG. 1 is a cross-sectional view showing a configuration of an apparatus used for carrying out the present invention. This apparatus includes a ladle 1 that contains molten steel M, a tundish 2 that receives molten steel M extracted from the bottom of the ladle 1, and a molten steel receiver 3 that receives molten steel M extracted from the bottom of the tundish 2. And.

  A cylindrical tundish nozzle 2 a made of alumina graphite and having an inner diameter of 15 mm is provided at the bottom of the tundish 2. The molten steel M in the tundish 2 can be extracted through the tundish nozzle 2a. Further, the tundish 2 is provided with a rod-shaped stopper 5 that opens and closes the opening of the tundish nozzle 2a from within the tundish 2. The stopper 5 is made of alumina graphite.

Further, the tundish 2 is provided with a porous nozzle 4. The porous nozzle 4 is made of ZrO ₂ graphite (85 mass% ZrO ₂ -5 mass% CaO-10 mass% C). The outer diameter of the porous nozzle 4 is 100 mm. There are many minute openings on the surface of the porous nozzle 4. The longitudinal direction of the porous nozzle 4 is oriented along the vertical direction. By supplying Ar gas from the upper end of the porous nozzle 4, Ar gas is discharged from a minute opening on the surface of the porous nozzle 4.

  The porous nozzle 4 is connected to the negative electrode of the DC power supply device 6, and the stopper 5 is connected to the positive electrode of the DC power supply device 6. Therefore, the stopper 5 functions as a counter electrode of the porous nozzle 4. A DC potential difference can be applied between the gas introduction pipe 4 and the tundish nozzle 2a by the DC power supply device 6.

  A load cell 8 is attached to the molten steel receiver 3. The total weight of the molten steel receiver 3 and the molten steel M in the molten steel receiver 3 can be monitored by the load cell 8, and therefore the weight of the molten steel M discharged from the tundish 2 into the molten steel receiver 3 can be known.

  In order to confirm the effect of the present invention, bubbles were generated in the molten steel M using the apparatus shown in FIG. 1 and the inclusion removal effect in the molten steel M was evaluated.

  First, using a high-frequency induction furnace (not shown), 1 ton of molten steel subjected to preliminary combined deoxidation with Si and Mn was melted. This molten steel M was poured from the high frequency induction furnace into the ladle 1 from above and transferred. Prior to pouring the molten steel M into the ladle 1, metal Al and Ti are charged in advance, and by injecting the molten steel M, Al and Ti are dissolved in the molten steel M, and the molten steel composition is shown in Table 2. Adjusted to those shown in. The balance of the molten steel M shown in Table 1 consists of Fe and impurities. In this composition, compared to steel having a general composition, Al and Ti-based oxides are easily generated in the molten steel M.

  Next, the tundish nozzle 2 a is closed by the stopper 5, the ladle 1 is moved above the tundish 2, and then moved into the tundish 2 through the nozzle provided at the bottom of the ladle 1. The transfer of the molten steel M in the ladle 1 was started. When the amount of molten steel in the tundish 2 increases, the stopper 5 is pulled up to fully open the tundish nozzle 2a and the amount of molten steel supplied from the ladle 1 is adjusted so that the amount of molten steel in the tundish 2 is always 300 kg. It was made to become. At this time, the superheat degree of the molten steel in the tundish 2 was in the range of 50 to 60 ° C.

The porous nozzle 4 was immersed in the molten steel M, and Ar gas was blown from the porous nozzle 4 into the molten steel M at a flow rate of 300 cm ³ / min (STP). Further, a potential difference was applied between the porous nozzle 4 and the stopper 5 by the DC power supply device 6, and a potential difference was applied between the molten steel M and the stopper 5.

  While the molten steel M is being discharged from the tundish 2, the weight of the molten steel M in the molten steel receiver 3 is monitored by the load cell 8, and the molten steel M is discharged into the molten steel receiver 3 except when the tundish nozzle 2a is blocked. When the total weight of the molten steel M reached 600 kg, the stopper 5 was lowered to close the tundish nozzle 2a, and the discharge of the molten steel M was stopped. Depending on conditions, before discharging 600 kg of molten steel M from the tundish 2, the tundish nozzle 2a was blocked.

  The above test was carried out under various conditions. The presence or absence of blockage of the tundish nozzle 2a and the time required to discharge 600 kg of molten steel M from the tundish 2 were measured, and the inclusion removal effect due to gas blowing into the molten steel M was indirectly evaluated.

  Table 3 shows the test conditions and results. Examples 1 to 4 are examples that satisfy the requirements of the present invention, and Comparative Examples 1 and 2 are examples that do not satisfy the requirements of the present invention.

  In each of Examples 1 to 4, 600 kg of molten steel M could be discharged in a short time. This is considered to be because the Ar gas blown in became fine and the oxide was efficiently removed. In particular, in Example 4, 600 kg of molten steel M can be discharged in the shortest time, and it is surmised that the oxide removal effect by Ar gas blowing was most remarkable.

  In Comparative Example 1, before discharging 600 kg of molten steel M, the tundish nozzle 2a was blocked. This is because Ar gas was blown into the molten steel M without applying a potential difference between the molten steel M and the porous nozzle 4, so that the bubbles of the Ar gas were not made fine, and the cleanliness of the molten steel M was almost improved. It is thought that there was not.

  In Comparative Example 2, 600 kg of molten steel M could be discharged, but compared with Examples 1 to 4, the time required for discharging was about 10 to 20% longer. It is considered that the progress of the blockage of the tundish nozzle 2a due to the inclusions was remarkable. Although a potential difference is given between the molten steel M and the tundish nozzle 2a, the contact angle of the molten steel M with respect to the tundish nozzle 2a does not become less than 90 °, and the bubbles of Ar gas in the molten steel M are refined. This is probably because the effect of removing oxide inclusions could not be lowered.

  Actually, as a result of investigating the clogging situation in the tundish nozzle 2a after the experiment, it was confirmed that a large amount of oxide inclusions were adhered in Comparative Examples 1 and 2 compared to Examples 1 to 4. .

1: ladle, 2: tundish, 2a: tundish nozzle,
3: molten steel receiver, 4: porous nozzle, 5: stopper,
6: DC power supply, 8: Load cell, M: Molten steel

Claims (2)

A method of generating bubbles in the molten metal by blowing an inert gas into the molten metal,
The contact angle of the molten metal with respect to the nozzle is controlled to less than 90 ° by applying a potential difference between the nozzle made of a conductive material having a gas blowing hole and the molten metal and energizing the molten metal. And a process of
A method for generating bubbles in the molten metal, the method comprising: blowing an inert gas from the gas blowing hole into the molten metal in a state where the contact angle is controlled to be less than 90 °. The bubble generation method according to claim 1,
The energized, the current density is adjusted between said with the molten metal nozzle to ^{35mA / cm 2 ~1A / cm 2} , the bubble generation method in the molten metal in.

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