JPH08143930A - Method for refining molten metal - Google Patents

Abstract

PURPOSE: To obtain molten metal high in cleanliness with a high refining efficiency by refining under low oxygen partial pressure atmosphere after decarburize-refining the molten metal and further refining under plasma heating at near the atmospheric pressure. CONSTITUTION: Solid raw material is charged into an induction heating refining furnace 5 and melted while reducing the pressure in an atmosphere shut-off chamber (a) and after blowing mixed gas of oxygen and Ar into the molten metal from a lance, the vacuum refining at <=1Torr is executed. At the time of completing the refining, the molten metal is poured in a re-refining vessel 27' with a cover body 27a and shifted to the re-refinzing position 27 with rails and a carriage 24 through a partition valve 4. Successively, slag-making agent is added from an auxiliary raw material charging system 9 and heated with an inert gas plasma heating device 23 to execute melting and heating of the slag-making agent, and Ar gas is blown and stirred from a porous plug 28 to execute the refining. At the time of completing the re-refining, the molten metal is cast into an ingot case 30 through a sliding nozzle 29. By this method, alloy high in cleanliness can be cast.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining a molten metal, which comprises decarburizing and refining a molten metal, subsequently refining it in a vacuum or in a low oxygen partial pressure atmosphere, and then refining it under plasma heating at around atmospheric pressure. It is a thing.

[0002]

2. Description of the Related Art In the refining method under vacuum or low oxygen partial pressure atmosphere (hereinafter referred to as refining under vacuum or low oxygen partial pressure),
It is widely used because high-level clean steel and the like can be easily obtained, and a vacuum induction heating furnace (also called VIF) is one of the means. In this refining under vacuum or low oxygen partial pressure, certain impurities become themselves or compounds such as oxides, and are evaporated, scattered or floated from the molten metal to be separated, whereby refining proceeds. Then, in this case, in the state where the refining has progressed, the impurities in the molten metal (impurities in the present invention are collectively referred to as substances causing impure elements or non-metallic inclusions) can be made to have a very low level.

In refining under vacuum or low partial pressure of oxygen, a part of vaporized and scattered components is condensed and adhered to the freeboard portion of the refining furnace in the form of scum. Further, on the surface of the molten metal in the refining furnace, another kind of non-evaporable dross-like impure component floats and is separated. When tapping the smelting furnace, the deposits and suspended matter are washed away by the molten metal stream and taken into the molten metal again, and then remixed in the molten metal. In addition, the refractory lining of the ladle or tundish that is the hot water container adsorbs the active gas components if it is not heated sufficiently before the hot water is received, and this refractory also has a vacuum or low temperature after the hot water is received. It partially reacts with the molten metal activated by the treatment under the partial pressure of oxygen, or is corroded by the molten metal, and the molten metal is contaminated by these active gas, reaction products, and eroded food.

The methods that have been tried up to now as methods for preventing re-mixing include tapping the bottom of a refining furnace and leaving a part of the molten metal in the furnace to prevent mixing of floating impurities. A method of discharging molten metal in the air with a furnace, and a method of removing substances that may be fine non-metallic inclusions with a ceramic filter in a ladle or a tundish have been proposed and partially put into practical use. Due to restrictions, it has not yet been widely adopted. Further, these methods are not effective for all remixed substances, pollutants and the like, and therefore it is necessary to combine them in order to make them effective for each remixed substance and the like. However, the more they are combined, the more costly the problem arises.

On the other hand, after vacuum refining, it is possible to remove re-mixed substances by refining with a ladle refining furnace of graphite electrode arc heating system, ASEA-SKF method, etc. Since it is graphite and its electrode is immersed in the slag on the surface of the molten steel, the problem of carbon pickup in the molten metal arises. Japanese Unexamined Patent Publication No. 4-318118 discloses a method in which a molten metal is decarburized by vacuum degassing, then plasma heated in a state of containing Sol.Al ≧ 0.2 wt% and stirred in the presence of a slag having a basicity of 8 or more. , A method for producing ultra-low carbon desulfurized and ultra-low sulfur steel is proposed.

In the above-mentioned method, since the slag before the plasma refining is brought into the plasma refining furnace, Sol.Al in the plasma refining must be increased to raise the basicity of the slag to 8 or more. That is, since slag is brought in, a large amount of a deoxidizing agent and a reducing agent are required for modifying the slag, so that there is a problem that the refining effect is inferior.

[0007]

By the way, in the products such as stainless steel, Ni-base alloys and Ni-Fe alloys, there are many metal materials for which the mixing of carbon is severely restricted for the reason of impairing the required properties. . Most of the above-mentioned carbon contamination is brought from the solid raw material at the time of melting, and the lower the oxygen content, the stronger the decarburization should be. Decarburization is a reaction with oxygen or the like, and means for burning it is known, and it is more efficient to combine it with a reduced pressure atmosphere or a vacuum atmosphere. However, in any case, the reaction product is an oxide, which adheres to the molten metal surface or wall of the smelting furnace and remixes into the molten metal depending on the movement of the molten metal, thus contaminating the smelted molten metal. It has a great nature. However, in order to avoid the above-mentioned decarburization treatment, it is necessary to select a high-grade material with a low C as the material to be inserted into the refining furnace, which significantly increases the raw material cost for melting, which leads to economical mass production. There are drawbacks that are not suitable.

The object of the present invention is to efficiently decarburize in the same preliminary refining furnace when refining stainless steel, Ni-based alloys, Fe-Ni alloys, etc., in which the carbon content of the above products is severely limited. Moreover, there is no problem of the above-mentioned carbon pickup, and it is not necessary to modify the carry-in slag,
It is an object of the present invention to provide a molten metal refining method capable of flexibly coping with high-level refining.

[0009]

According to the present invention, an oxygen source or a mixture of an oxygen source and an inert gas is added to or blown into a molten metal melted in a container having a heating means to react with carbon in the molten metal to form CO. A predetermined level of pre-refining is achieved by gasifying and releasing out of the vessel, followed by refining and deoxidizing in a vacuum or low oxygen partial pressure atmosphere. Then, in preliminary refining, it was once separated from the molten metal, but it still remained in the container, and impurities existing as scum-like, dross-like, etc. were added to the container as it was, and a new slag-making agent was added. Refining that efficiently removes impurities and non-metallic inclusions caused by the scum and dross mixed in the molten metal by plasma refining while preventing re-mixing of carbon by performing plasma heating, or refining to a higher level Is the way. Further, the present invention, after pre-refining,
The molten metal can be transferred to another container for re-refining and then re-refined.

That is, the first aspect of the present invention is to add or blow an oxygen source or a mixture of an oxygen source and an inert gas into a molten metal in a container having a heating means, and after adding or blowing the oxygen source, vacuum or A method for refining a molten metal, which comprises refining by adding a slag-making agent to the molten metal after performing other preliminary refining in a low oxygen partial pressure atmosphere and heating the molten metal with an inert gas plasma, The invention is to add or blow an oxygen source or a mixture of an oxygen source and an inert gas into a molten metal in a container having a heating means, and after adding or blowing the oxygen source, in a vacuum or a low oxygen partial pressure atmosphere, and perform another preliminary operation. After the refining, the molten metal is transferred to a container different from the container, a slag-forming agent is added, and the mixture is heated by an inert gas plasma to be refined again. The feature of the present invention resides in that the oxidative refining with an oxygen source is carried out before the pre-refining, and then the pre-refining and the re-refining with an inert gas plasma are combined.

The term "vacuum or low oxygen partial pressure atmosphere" as used in the present invention means an atmosphere of less than atmospheric pressure or an oxygen partial pressure of atmospheric air, that is, an oxygen partial pressure atmosphere of less than 213 HPa (1013 HPa × 0.21). Means for satisfying this condition include exhausting with a vacuum pump to reduce the pressure, and inert gas (A
There is a method of substituting a part of oxygen with r or N ₂ gas) or by combining reduced pressure and gas substitution to obtain an inert gas atmosphere of several 100 Torr or less. As a practical furnace for these, there are a vacuum induction furnace (VIF), an AOD furnace, a VOD furnace, etc. desirable. Further, in the second invention, it is preferable that the renewal container is prepared so as not to be contaminated by splash or fume in the pre-refining. A pre-smelting furnace and re-smelting furnace combination apparatus suitable for this is shown in FIG. In FIG. 2, the renewal container is set in advance in a second atmosphere blocking chamber that is connected to the atmosphere blocking chamber a via a partition valve 4.

That is, since the pre-refining is carried out in the atmosphere shut-off chamber a under a vacuum or low oxygen partial pressure atmosphere, the re-refining vessel 27 is preheated and then in the vacuum or depressurized chamber connected via the partition valve 4. Be prepared. When the refining in the atmosphere shut-off chamber a is completed, the sluice valve 4 is opened, and the re-refining vessel 27 is moved to the position 27 'of the air shut-off chamber a by the rail and the carriage 24 through the opening. At this time, the atmosphere shut-off chamber a and the room in which the re-refining vessel is set communicate with each other, but both are in a vacuum or decompressed state, so that only the same atmospheric pressure is obtained, and a vacuum or low oxygen partial pressure atmosphere can be maintained. become. In this way, the renewal container is outside the atmosphere shut-off chamber a during the pre-refining, so that it is not contaminated with splash or fume.

Further, in the refining method and apparatus of the present application, re-refining is performed by a porous plug or other method.
It is desirable to perform it under stirring conditions such as the gas injection method such as r and the induction method, and the tapping method or stopper method is used for tapping the hot water from the re-refining vessel rather than the slanting method or stopper type. It is suitable because there is no inconvenience such as contamination of the metal or damage to the stopper due to plasma heating.

[0014]

As described above, according to the method of the present invention, stainless steel, Ni-based alloy or Ni-Fe having a particularly low carbon content is used.
Suitable for refining alloys, etc.
A gas mainly composed of oxygen gas, which is an example of an oxygen source, is blown into the molten metal to cause sufficient oxygen and carbon to react and be discharged as CO gas outside the container, and then the oxygen remaining in the molten metal is vacuumed or reduced. Preliminary refining is performed by releasing it into an oxygen partial pressure atmosphere. As a result, the generation of slag is relatively small, the carbon in the molten metal is low, and the residual oxygen level is low, and the slag forming agent that is newly added during re-refining by plasma heating, which is the next important step, Rerefining can be performed effectively by reducing the types and amounts.

Speaking of the advantages of the pre-refining and the subsequent re-refining, in the present invention, the carbon mixed during the melting is removed by the pre-refining without generating the slag, so that a substantially clean state without slag is obtained. You can move on to the next plasma refining. This is an important step of the present invention, re-refining by plasma heating can be specialized in removing impurities such as scum and dross during pre-refining, and the type and amount of slag forming agent used under plasma heating There is an advantage that it can be small.

Although blowing of a gas mainly composed of oxygen gas, which is carried out during the above-mentioned pre-refining, is a desirable method for decarburizing, as another method, blowing of oxygen gas and a main component or a sub-component of the molten metal component are used. By adding an oxide composed of the main component (for example, in the case of Fe-Ni alloy, nickel oxide can be used as an oxide) and oxidizing the carbon in the molten metal, the carbon in the molten metal can be further removed. It may be removed. In this case, since the primary decarburization reaction has been completed in advance by blowing in a gas mainly containing oxygen, the addition of the oxide can be small, which is a preferable method. According to this method, the generation of slag due to the addition of oxides is small, and a part of the slag is decomposed into gas in the subsequent vacuum refining and is discharged to the outside of the container. Therefore, the slag modification due to the addition of oxides can be almost ignored for the slag-forming agent that is newly added under the plasma heating which is the next step. Further, by carrying out decarburization refining by blowing gas mainly composed of oxygen gas at the time of preliminary refining of the present invention, there is an advantage that denitrification is simultaneously promoted in the equilibrium reaction.

The re-refining in the present invention effectively absorbs impurities and non-metallic inclusions once separated in the preliminary refining step by an activated slag-forming agent which is newly added under the heating of an inert gas plasma. It is carried out by That is, in the present invention, since the newly added slag-forming agent has substantially no slag at the time of pre-smelting, a new slag-forming agent for modifying the brought-in slag from the pre-smelting is not required, It is not necessary to raise the basicity of the slag-making agent added to the composition more than necessary, and a small amount may be added. The slag forming agent is Ca
A compound flux centered on O with a basicity of 2 or more. It is optimal to adjust it to 3 to 7. As a result, it is sufficiently heated by the inert gas plasma heating to have low viscosity and activation, so that the contaminants caused by the re-mixed substances such as the above-mentioned deposits, suspended substances, non-metallic inclusions and refractories can be effectively Can be collected, preferably by sufficient agitation such as by blowing an inert gas described later,
Contact with the molten metal while exchanging them sufficiently, producing a high refining effect.

It should be noted that the fact that the molten metal at the end of the pre-refining before plasma refining is substantially slag-free means that a slag-forming agent is added for the purpose of pre-refining to complete the reaction. It means that there is no slag that has occurred. Therefore, in the second invention of the present invention, adding the slag forming agent to the molten metal immediately before or after the completion of the pre-smelting before pouring the molten metal into the re-smelting in another container, and pouring the molten metal into the re-smelting furnace. However, since this is not intended for refining by adding a slag-forming agent in the pre-refining furnace, it is substantially the same as the one without slag for reaction in the pre-refining, It is included. Further, the inert gas plasma heating method carried out in the atmosphere effectively heats the slag-forming agent as described above, while coating the surface of the molten metal or the slag-forming agent to prevent oxidation of the molten metal or the slag-forming agent. While preventing oxidization, the temperature of the molten metal is raised or the temperature drop is compensated, and there is no risk of re-contamination such as carbon pickup as in the graphite electrode arc heating method. The method of the present invention includes a method of refining to a higher level than the pre-refining as in Examples described later.

As a desirable mode in the re-refining method by plasma heating which is the refining in the final step of the present invention, stirring may be added. The stirring method is preferably an inert gas from a porous plug provided at the bottom of the furnace from the viewpoint of the stirring effect. Moreover, you may use an electromagnetic stirring device individually or in combination. As described above, the inert gas plasma heating device does not generate an oxidizing gas such as CO ₂ , H ₂ O, and free O ₂ like the burner, and is suitable for high temperature heating. Therefore, the inert gas plasma heating device is used for re-refining in the present invention. Of course, this can be used to preheat the re-refining vessel to a high degree of cleanliness and high temperature while continuing the pre-refining when renewing the vessel, while using this heating device.

[0020]

【Example】

(Example 1) Using the equipment shown in FIG. 1, operation was performed in the following procedure. After the solid raw material was charged into the induction heating and refining furnace 5, the lid 3 was applied, the inside of the atmosphere blocking chamber a was depressurized by the vacuum exhaust system 6a, and the melting was started by the induction heating furnace. Next, a lance (not shown) was inserted into the molten metal, and a mixed gas of oxygen gas and Ar gas was blown from the outside of the atmosphere blocking chamber a through the lance. After the blowing of the mixed gas was completed, the lance was pulled up from the molten metal to stop the gas supply, and subsequently, the atmosphere blocking chamber was vacuum refined to 1 Torr or more for vacuum refining.

In parallel, inside the atmosphere shut-off chamber a, a re-refining container preheated in advance is set with a container lid 27a. This is to prevent splashing during pre-refining. When the refining in the atmosphere shut-off chamber a is completed,
By tilting the induction heating refining furnace 5, the molten metal is discharged to the re-refining vessel 27 '. Next, the partition valve 4 is opened, and the re-refining vessel 27 ′ is moved to the re-refining position 27 by the rail and the carriage 24 through the opening. Subsequent raw material input system 9
By adding a slag forming agent by, heating by an inert gas plasma heating device to melt and heat the slag forming agent, and by stirring by Ar gas blowing from the porous plug 28 through the inert gas introducing system 7b, Re-refined. After the re-refining was completed and a predetermined sedation was performed, it was cast into the ingot case 30 through the sliding nozzle 29. In this embodiment, the re-refining vessel 27 'is set in advance in the air shut-off chamber a, but it may be carried out by the equipment as shown in FIG. In this case, it is possible to sufficiently preheat the re-refining vessel and to prevent contamination such as splash.

Next, the effects of the method invention of the present application will be described with various test examples. In each case, the molten metal used in the experiment was Fe-
42Ni alloy. FIG. 3 shows the elapsed time from the time when the solid raw material was charged in the vacuum induction heating and refining furnace to start the melting and the melting of the raw material was completed, and the evacuation was continued without adding or blowing the oxygen source. It is the figure which showed the result of having measured the progress degree of preliminary refining by the O value and C value in molten metal. From FIG. 3, the O value in the molten metal rapidly decreases due to the vacuum refining, and at that time, oxygen and carbon are combined to form CO gas, which is discharged to the outside of the atmosphere shut-off chamber a. As a result, the C value simultaneously decreases. You can see that However, the C value reduced by the above-mentioned preliminary vacuum refining was at most about 60 ppm.

FIG. 4 shows that the molten metal refined under the same conditions as the preliminary refining is poured into the re-refining container in which the refractory lining has been heated in advance by an inert gas plasma heating device by decanting under the same conditions as described above, and the slag forming agent. Fig. 3 shows a change in gas concentration when re-refining is performed in the present invention while adding argon, stirring argon gas with a porous plug at the bottom of the furnace, and heating an inert gas plasma from the upper surface of the molten metal. From this figure, re-melting of the re-mixture was prevented by the re-refining according to the present invention, the gas concentration rather decreased gradually with the passage of time, and the refining progressed above the level of the pre-refining. I understand. However, the C value was almost the same as the level after the pre-refining, and it could not be further decarburized by the re-refining.

Next, in FIG. 5, the solid raw material is melted in the induction heating furnace 5 in the same manner as described above, and when the raw material is completely melted, a lance is inserted into the molten metal, and from this lance, oxygen gas and Ar gas are mixed. The behavior of O value and C value was shown when the gas injection was started and nickel oxide was added in an amount of 3 kg per ton of molten metal, followed by pre-refining by increasing the degree of vacuum. From FIG. 5, the decarburization reaction can be promoted more by blowing the mixed gas and adding nickel oxide than in the case of FIG. 3, but the O value of the blowing of the mixed gas is once increased by the addition of nickel oxide, and the subsequent vacuum Although the O value decreased by refining, it was difficult to sufficiently reduce the O value only by vacuum refining because an oxygen source was added, and the O value after refining became higher than that in FIG. However, the O value can be further reduced by combining re-refining in the subsequent plasma refining furnace, which is an important constituent feature of the present invention.

FIG. 6 shows that the molten metal after blowing the mixed gas shown in FIG. 5 and adding nickel oxide to accelerate decarburization and subsequently performing vacuum refining is transferred to a plasma refining furnace. 9 is a plot of changes in O value and C value in the molten metal with respect to the time elapsed from the time when the slag forming agent was added. The refining method in the plasma refining furnace is the same as in the case of FIG. 4, but deoxidation can be sufficiently performed in a state where the C value hardly rises, and impurities and nonmetallic inclusions can be removed. I understand that As mentioned above, in order to refine alloys that require particularly low carbon, a sufficient decarburization reaction is performed in the preliminary refining furnace, and the clean molten metal deoxidized by vacuum refining is re-heated by the next plasma heating. It can be seen that refining yields an extremely clean alloy.

Although the vacuum refining method is used as an example of the pre-refining in the above embodiments, the present invention is not limited to this. In other words, depending on the alloy component elements contained in the molten metal that is the object of refining, in order to prevent loss due to evaporation of the components, it is usually 200 Torr at absolute pressure.
An inert gas atmosphere of a certain degree or less is appropriately selected, and the present invention includes the case where the vacuum is not high. Further, in the above-mentioned embodiment, the melting was performed directly from the solid raw material in the preliminary refining furnace, but the step before the vacuum refining, that is, the melting and the blowing or addition of the oxygen source may be performed in another container. Needless to say.

(Embodiment 3) Using the equipment shown in FIG. 7, operation was performed in the following procedure. Fe-same as in Example 1
After the solid raw material of 42Ni alloy was charged into the container 50, the container vacuum lid 53 was provided, the pressure inside the container 50 was reduced by the vacuum exhaust system 54, and the melting was started by the induction heating coil 52. Next, a lance (not shown) was inserted into the melt 51, and a mixed gas of oxygen gas and Ar gas was blown from the outside through the lance. After the blowing of the above mixed gas is completed, subsequently, a total of 3 kg of nickel oxide and iron oxide (Fe ₂ O ₃ ) is added to the molten metal per ton of the charging device 55.
Added from.

Next, the capacity of the vacuum exhaust system 54 was increased to start deoxidation. When the deoxidation refining was completed, the vacuum evacuation system 54 was stopped, and argon gas was introduced from the argon bottom-blown stirrer 58 to replace it, and then the slag forming agent was added from the charging device 55. Next, the plasma heating torch 56 set outside the container vacuum lid 53 is inserted into the container 50 to start plasma heating, and at the same time, the argon bottom blowing stirring device 5 is used.
The molten metal was stirred while blowing argon gas from No. 8, and refining was performed. When the re-refining was completed, the sliding nozzle 57 was opened and the hot water was received in the ingot case prepared under the sliding nozzle. As a result of gas analysis of the sample collected from the ingot thus obtained,
The O value of the alloy obtained by decarburizing and deoxidizing by adding the mixed gas and nickel oxide of Example 1 and further refining in the plasma heating furnace is 22.
The alloy by the above method had an O value of 26 ppm and a C value of 29 ppm, while the ppm and C values were 26 ppm.

[0029]

As described above, the present invention uses a refining furnace having a heating means, and supplies sufficient oxygen by supplying an oxygen source such as blowing in oxygen gas or a mixed gas of oxygen gas and an inert gas. In addition, by further refining and deoxidizing in a vacuum or low oxygen partial pressure atmosphere, it is possible to move to the next plasma heating refining in the state of relatively clean molten metal with substantially no slag, so refining efficiency is extremely high. An alloy with high cleanliness can be obtained. According to the method of the present invention, a molten metal having a relatively small amount of slag generated in the pre-refining, a low carbon content in the molten metal, and a low residual oxygen level can be obtained.
As a result, it is possible to effectively carry out re-refining by reducing the kind and amount of the slag-forming agent newly added at the time of re-refining by plasma heating.

Further, by carrying out decarburization refining by gas blowing mainly of oxygen gas at the time of preliminary refining of the present invention,
The equilibrium reaction also has the advantage of simultaneously promoting denitrification. Also,
According to the present invention, the molten metal which has been subjected to the pre-refining for decarburization and deoxidation is replaced with another container, so that the re-refining by the next plasma heating can be efficiently performed in a state of a more clean molten metal.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an apparatus used in an example of the present invention.

FIG. 2 is a diagram showing an example of an apparatus used in an example of the present invention.

FIG. 3 is a diagram showing a change in C value with respect to an elapsed time from when a nozzle is inserted into a molten metal melted in a vacuum induction furnace and a mixed gas of oxygen gas and Ar gas is started to be blown.

[Fig. 4] After the above pre-refining (Fig. 3), the molten metal is poured into a re-refining furnace in another container, a new slag-making agent is added, and re-refining by plasma heating is started. It is a figure which shows the change of a value and C value.

FIG. 5 is a diagram showing changes in O value and C value with respect to elapsed time from the time when nickel oxide is added after the blowing of the mixed gas of oxygen gas and Ar gas is completed in the vacuum induction furnace. .

[Fig. 6] After the preliminary refining (Fig. 5), the molten metal is poured into a re-refining furnace in a separate container, a new refining agent is added, and re-refining by plasma heating is started. It is a figure which shows the change of a value and C value.

FIG. 7 is a diagram showing an example of an apparatus used in Examples of the present invention.

[Explanation of symbols]

1 vacuum or low oxygen partial pressure refining equipment, 2 atmosphere blocking chamber body, 3 lid, 4 partition valve, 5 induction heating refining furnace, 6a vacuum exhaust system, 6b vacuum exhaust system, 7a inert gas introduction system, 7b inert Gas introduction system, 8 auxiliary raw material charging system, 9 auxiliary raw material charging system, 20 re-refining device, 22 partition door, 23 inert gas plasma heating device, 24
Rails and carts, 27 re-refining vessels (re-refining position), 2
7a Container lid, 28 Porous plug, 29 Sliding nozzle, 30 Ingot case, 31 Carriage, a Air shutoff chamber, b Air shutoff chamber, 50 container, 5
1 molten metal, 52 induction heating coil, 53 container vacuum lid, 5
4 vacuum exhaust system, 55 charging device, 56 plasma heating torch, 57 sliding nozzle, 58 argon bottom blowing stirring device

Claims (3)

[Claims] 1. An oxygen source or a mixture of an oxygen source and an inert gas is added or blown into a molten metal in a container having a heating means, and after the addition or blowing of the oxygen source, a vacuum or a low oxygen partial pressure atmosphere, A method for refining a molten metal, which comprises performing a preliminary refining process and then adding a slag forming agent to the molten metal and heating the molten metal with an inert gas plasma for refining. 2. An oxygen source or a mixture of an oxygen source and an inert gas is added to or blown into the molten metal in a container having a heating means, and after the addition or blowing of the oxygen source, a vacuum or a low oxygen partial pressure atmosphere is used. After the preliminary refining, the molten metal is transferred to a container different from the container, a slag-forming agent is added, and the mixture is heated by an inert gas plasma to be refined again. 3. The molten metal refining method according to claim 1 or 2, wherein the heating means for the pre-refining is induction heating.