JPH08143937A - Method for refining molten metal - Google Patents

Abstract

PURPOSE: To provide an effective and new refining method by which impurity elements can thoroughly be reduced. CONSTITUTION: In this refining method of molten steel, the molten steel obtd. in a primary melting are refining furnace, is subjected to the removal of slag, successively heated inn a vessel having a heating means such as a vacuum induction furnace 5, also pre-refined under reducing pressure and <=300ppm oxygen atmosphere, the added with slag-making agent, and heated by a gas plasma to execute re-refining. It is desirable that the shifting period from the pre-refining to the re-refining is kepet at <=300ppm oxygen atmosphere. Further, it is desirable that the re-refining is executed in the vessel separated from the pre-refining.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal refining method effective for reducing impurity elements.

[0002]

2. Description of the Related Art In a refining method under a reduced pressure atmosphere, it is widely used because it is possible to obtain a high level of clean steel.
Refining by a vacuum induction heating furnace (also called VIF) is one of the means. In refining in a reduced pressure atmosphere, by heating the molten metal exposed under reduced pressure, gas components such as H ₂ , O ₂ , and N ₂ , carbon, Zn, and Zn,
Low-boiling point elements such as Pb that reduce the strength of steel and low-boiling point oxides that are the origin of oxide inclusions such as MnO and SiO ₂ are separated by evaporation. In some cases, NiO
In some cases, the addition of FeO or Fe ₂ O ₃ may accelerate the decarburization of the molten metal. In particular, refining by a vacuum induction heating furnace, which is performed on molten metal that has undergone primary melting or refining in a steelmaking furnace such as an electric arc furnace or converter, does not add oxygen fixing elements such as Al to the molten metal, or adds a very small amount. Since refining proceeds by this, it is possible to obtain clean steel with very few inclusions, which is an effective refining method for molten steel.

Recently, JP-A-4-318118 and JP-A-5-1886814
In No. 1, the molten metal from the steelmaking furnace is decarburized by vacuum degassing treatment, specifically, the RH method, and then plasma heating is performed and quicklime or the like is added as a flux to adjust the basicity of the slag to 7 or A method of obtaining ultra-low carbon and ultra-low sulfur steel by adjusting the slag to 8 or more and performing slag reduction with aluminum has been proposed. According to this method, even when the slag for steelmaking is mixed, the slag is reformed by adjusting the basicity of the slag by plasma heating and quicklime, and desulfurization is promoted. It is said that there is no carbon pollution due to impurities and that impurities can be reduced.

[0004]

In refining in a reduced pressure atmosphere using the above-mentioned vacuum induction heating furnace or the like, since a slag-forming agent for removing impurity elements is not added, the problem that the addition of the slag-forming agent increases impurities. There is no. However, in refining in a reduced pressure atmosphere, some of the impurities evaporated and scattered from the molten metal condense on the furnace wall portion that is not in contact with the molten metal of the furnace during the refining period. Further, on the surface of the molten metal in the refining furnace, a compound of impurities such as Al ₂ O _{3 that} cannot be vaporized remains in the state of so-called scum. When such impurities adhering to the furnace wall or floating compounds of impurities are discharged from the refining furnace, they may be re-introduced into the molten metal and remixed in the molten metal. In addition, after refining under reduced pressure, if the atmospheric pressure is released as it is, the compound containing suspended impurities may become non-metallic inclusions and may be redissolved in the molten metal.

A method that has been attempted up to now as a method for preventing the above-mentioned mixing of impurity elements after refining under reduced pressure is to perform tapping of the bottom of a refining furnace to obtain a molten metal portion containing suspended matter containing impurities. A method of leaving the molten metal in the furnace, a method of discharging the molten metal in the air with an induction skull furnace,
In a ladle or a tundish, a method of removing fine nonmetallic inclusion-causing substances with a ceramic filter has been proposed and partially put into practical use, but due to various restrictions, it has not yet been widely adopted. . After vacuum refining, graphite electrode arc heating ladle refining furnace, ASE
It is possible to remove the re-mixed substances by refining by the A-SKF method or the like, but these methods cause a problem of carbon pickup from the electrode into the molten metal.

On the other hand, the above-mentioned Japanese Patent Laid-Open Nos. 4-318118 and
In 5-186814, basically, impurities are reduced in the state where slag for steelmaking remains. Therefore, in the desulfurization process by plasma heating, it is necessary to add a large amount of Al as a reducing agent for slag for steelmaking and a large amount of flux such as quick lime for adjusting to high basicity.

However, as a recent demand for clean steel, the inclusion of a large amount of Al in the steel causes hard fine inclusions and deteriorates the surface properties, punchability and etching quality of a polished steel plate having a high rolling ratio. In steel bars, Al is an element that forms continuous Al ₂ O ₃ -based non-metallic inclusions and deteriorates mechanical properties, so it has been required to avoid it as much as possible. In addition, adding a large amount of flux for the purpose of residual slag for steelmaking and reforming of slag means addition of components that may be impurities in the steel, so that the cleanliness of the steel refined in a reduced pressure atmosphere can be improved. It may decrease. The present invention has been made in view of the above problems, and an object thereof is to provide an effective new refining method capable of thoroughly reducing impurity elements.

[0008]

The present inventors SUMMARY OF THE INVENTION, to the molten metal, by performing a preliminary refining pressure atmosphere, for example H _2, O _2, N _2, such gas components, carbon and Zn, P
b and other low boiling point elements that reduce the strength of steel, such as MnO and S
The low boiling point oxide, which is the origin of oxide inclusions such as iO _{2, is} reduced as much as possible by evaporation separation, and the floating materials on the molten metal such as Al ₂ O ₃ which cannot be separated and the refractory atmosphere are refined to the furnace wall. As a method for removing a small amount of re-mixed substances originating from adhering impurities, it is effective to carry out preliminary refining in a specific atmosphere and perform plasma heating under the slag forming agent for the molten steel. I found a thing. Then, as a result of studying further steel cleaning, a small amount of slag mixed from the primary melting / smelting furnace significantly reduces the refining effect in pre-smelting, so it is necessary to strictly limit the mixing of slag from the primary melting / smelting furnace. Therefore, the present invention has been reached.

That is, according to the present invention, the molten steel obtained in the primary melting and refining furnace is slag-removed, then heated by a container having a heating means, and pre-refined under reduced pressure and an atmosphere of 300 ppm or less of oxygen. This is a refining method for molten metal, which comprises adding a slag forming agent to the molten metal, heating it with gas plasma and refining it again.

In the present invention, it is preferable to maintain an atmosphere of oxygen of 300 ppm or less during the transition period from preliminary refining to re-refining. The heating in the pre-refining is preferably induction heating. Further, in the present invention, it is desirable to carry out re-refining in a container different from the pre-refining.

[0011]

As described above, a major feature of the present invention is that the molten metal from the primary melting / refining furnace is first slag removed. In the present invention, the molten metal obtained in the primary refining / melting furnace is subjected to preliminary refining in a reduced pressure atmosphere,
_2, O _2, N ₂ gas, such as, Zn, low boiling elements to reduce the strength of the steel, such as Pb, by impurities and low boiling oxides of origin of oxide inclusions such as MnO and SiO ₂ evaporates separating It is to be reduced as much as possible.

However, when the above-mentioned pre-refining is attempted in the state where the slag of the primary melting and refining is present in the molten metal obtained in the primary melting and refining furnace such as an electric arc furnace or a converter, it is exposed to a reduced pressure atmosphere. Since the surface of the molten metal to be coated is covered with slag, there is a problem that the depressurizing effect on the molten metal cannot be obtained and refining does not proceed. In addition, the slag of the primary melting and refining,
In the pre-refining under reduced pressure, it reacts with the furnace wall of the pre-refining furnace, causing the refractory to melt and increase non-metallic inclusions in the molten metal.

Therefore, it is necessary to remove the slag in the primary melting and refining before the preliminary refining. As the slag removing method in the present invention, in addition to the relay dle method, a method of scraping the floating slag after pouring the hot water into a ladle can be used. Of course, in the present invention,
It is most desirable not to mix slag. Even when it is difficult to completely remove slag, the amount of slag mixed in the pre-refining is preferably 0.5 wt% or less based on the weight of the molten metal to be pre-refined. More preferably, it is 0.1 wt% or less.

According to the present invention, after the above-mentioned slag is removed, preliminary smelting is carried out by heating the molten metal in a reduced pressure atmosphere, and a small amount of impurities, which cannot be separated and removed by the preliminary smelting alone, are produced. There is an important feature in capturing and removing by refining by adding slag agent and heating by gas plasma. As described above, the pre-refining, which is one of the important requirements of the present invention, is to remove H ₂ , O ₂ ,
Gas components such as N _2, low-boiling elements such as Zn and Pb that lower the strength of steel, and low-boiling oxides that are the origin of oxide inclusions such as MnO and SiO ₂ are reduced as much as possible to reduce Al ₂ A compound that causes inclusions of steel such as O ₃ is floated above the molten metal.

In the depressurized vessel having a heating source according to the present invention, a furnace satisfying the conditions of oxygen atmosphere of 300 ppm or less is a VOD for burning carbon in molten steel using oxygen as a heating source to heat the vacuum induction furnace. Can be given. The molten metal preliminarily refined in these refining furnaces is transferred to the same container or another container and then heated by gas plasma to be refined again. When plasma heating is also performed in the same furnace,
It is important to add a new slag-making agent after removing the slag generated in the pre-refining as much as possible. In this respect, the combination of the vacuum induction furnace and the plasma heating furnace is one of the most desirable combinations. This is because, in a vacuum induction furnace, basically, no slag-forming agent is used, so the target of contaminants is limited to the suspended matter on the molten metal and the impurities adhering to the furnace wall. . Such so-called scum-like contaminants
It can be easily removed by gas plasma heating in the presence of a slag forming agent, which is a conventional process of the present invention, and does not particularly require addition of a large amount of quicklime or a large amount of Al.

Preliminary refining performed in a vessel having a heating source specified in the present invention under reduced pressure and an atmosphere of oxygen of 300 ppm or less has the purpose of the vacuum degassing method represented by the RH method in that refining is performed. It's completely different. That is, the pre-refining of the present invention is a refining that can be independently performed only if refining of steel having extremely few impurities is required, and impurities in the molten metal to be sent to the next step are reduced to an extremely small amount. Is. On the other hand, in the simple vacuum degassing treatment, attention is focused only on the removal of gas components, and the steelmaking slag may be mixed in the next step, which cannot be said to be sufficient refining. In the vacuum degassing process, non-gaseous impurities such as steelmaking slag are removed for the first time in refining in the next step.

In the present invention, the conditions for pre-refining are defined by a container having a heating source under a reduced pressure and an atmosphere of oxygen of 300 ppm or less. This is because it is essential to have a heating source in order to carry out pre-refining as in the present invention. And in the present invention, the atmosphere is a reduced pressure atmosphere lower than atmospheric pressure in order to proceed with the removal of gas components and low-boiling impurities, and 300 ppm or less in order to proceed with refining while suppressing the formation of oxides, preferably It is necessary to create an atmosphere with an oxygen concentration of 100 ppm or less. The pressure of the atmosphere applied to the present invention is preferably 100 Torr or less.

Another important requirement of the present invention is that a smelting agent is further added to the highly clean molten metal obtained by the above-mentioned pre-smelting, and the smelting agent is heated by gas plasma. . As a result, a small amount of impurities such as oxides such as alumina with oxygen fixed in the molten steel and sulfides with sulfur fixed which cannot be removed by the pre-refining can be floated and captured in the slag forming agent on the molten metal. In addition, impurities that could not be removed by preliminary refining are floated and captured in the molten metal by the slag-forming agent. Since the re-refining of the present invention is applied to the molten metal subjected to the pre-refining, a small amount of the smelting agent to be added is effective, and it is possible to reduce the contamination of the molten metal by the smelting agent as much as possible. is there. Examples of the slag forming agent used in the present invention include CaO and C
A slag forming agent containing aF or Al ₂ O ₃ as a main component can be used. Of course, a mixture of these may be used.

The gas plasma heating method effectively heats the smelting agent while at the same time coating the surface of the molten metal or the slag forming agent to prevent the molten metal from being oxidized or the oxidizing agent to become oxidizable. There is an advantage that the temperature is raised or the temperature is reduced, and there is no risk of recontamination such as carbon pickup or fluctuation of carbon amount as in the graphite electrode arc heating method. As the gas plasma used in the present invention, an inert gas plasma,
Nitrogen gas plasma, inert gas + hydrogen gas plasma, etc. can be applied.

It is effective that the re-refining in the present invention is started as early as possible after the preliminary refining. Impurities such as suspended solids will dissolve in the molten metal over time, so if you capture them with a slag-making agent before they dissolve, re-smelting can be completed without spending extra refining time. is there. Therefore, when re-refining is performed in another container, it is installed adjacent to the pre-refining furnace, and the molten metal can be moved in the same refining device through, for example, a partition valve, and the re-refining furnace by plasma can be quickly used. Equipment that can be moved to The equipment shown in FIG. 1 in the preferred embodiment is the preferred form. Further, in the present invention, the re-refining is preferably carried out under stirring conditions such as a gas injection method or an induction method by a porous plug or other method, and tapping from the re-refining container is a sliding provided at the bottom of the container. The nozzle method is more suitable than the tilting method and the stopper method because they do not have the disadvantages such as mixing of slag and damage to the stopper due to plasma heating.

As described above, the transition period from the pre-refining to the re-refining in the present invention is preferably maintained in an atmosphere of oxygen of 300 ppm or less, preferably 100 ppm or less. This is to prevent the molten metal surface activated by preliminary refining from being oxidized and producing new impurities. In addition, in the present invention, induction heating melting is an effective means for heating the molten metal without requiring means such as gas blowing into the molten metal, and it is desirable that the heating in the pre-refining is by induction heating.

The pre-refining and re-refining in the present invention are
Although it is possible to use the same container, it is effective to carry out the operation in separate containers. Since the smelting agent is used in the re-smelting of the present invention, if the container used for the pre-smelting is to be applied to the re-smelting as it is, the floating impurities captured by the smelting agent after the re-smelting remain in the container. It takes a lot of time to clean the container for the next preliminary refining,
This is because it is not preferable.

In the present invention, as described above, a refining device provided with a container for pre-refining and a container for re-refining container for applying gas plasma separately, or for pre-refining and re-refining for applying gas plasma A refining device in which the same container is used can be applied. When separate containers are used for pre-refining and re-refining, as an example, the apparatus shown in FIG. 1 can be applied. In FIG. 1, as a pre-refining apparatus 1 for applying pre-refining by applying a reduced pressure atmosphere, an atmosphere shutoff chamber main body 2 and a lid 3 each having a partition valve 4 on a side wall.
Atmosphere shut-off chamber a, an induction heating and refining furnace 5 provided in the air shut-off chamber a, and a vacuum exhaust system 6a including a valve.
And an inert gas introduction system 7a including a slag forming agent or an auxiliary raw material such as an alloy (for induction heating and refining furnace) and a valve
It consists of and.

The re-refining device 20 has a partition door 22 for maintenance on one end side, and is connected to the atmosphere shut-off chamber a via a shut-off valve 4, and an atmospheric shut-off chamber b and an atmospheric shut-off chamber b are provided. Directly below the gas plasma heating device 23 from the hot water receiving position (indicated by 27 ') by tilting the induction heating and refining furnace 5 provided in the preliminary refining device 1 and the gas plasma heating device 23 that can be moved up and down on the ceiling part of the Re-refining vessel 27 having a porous plug 28 and a sliding nozzle 29 at the bottom of the vessel, which is movable by a rail and a truck 24 through an opening of the valve 4 up to a re-refining position (shown by 27) in the section, and a valve. Is an apparatus including an exhaust system 6b including the above, an inert gas supply system 7b including a valve, a casting ingot case 30, and a carriage 31 thereof.
In addition, refining using the apparatus of FIG. 1 will be described in detail in Examples described later.

Further, in the case of using the same container for the pre-refining and the re-refining, as a concrete example, the apparatus shown in FIG. 2 can be applied. FIG. 2 shows an apparatus configuration in which pre-refining and re-refining are performed in the same container 50. The refining apparatus of FIG. 2 shows a state in which a container 50 holding a molten metal 51 in a state where slag has been removed from a primary refining / melting furnace is inserted into an induction heating coil 52 connected to a power source. The pre-refining is performed by heating the molten metal 51 with the induction heating coil 52 while the container vacuum lid 53 is installed and the vacuum exhaust system 54 maintains a predetermined reduced pressure atmosphere. After the predetermined preliminary refining is completed, a smelting agent is added from the charging device 55 and the atmosphere is replaced so as to be a gas atmosphere suitable for gas plasma, and then a plasma heating torch 56 for melting and heating the smelting agent is used. Insert and perform refining. In the re-refining, the molten metal is stirred by the argon bottom blowing stirring device 58. After the re-refining is completed, the molten nozzle 51 is opened in the sliding nozzle 57 and cast in a predetermined ingot case or the like to complete the pre-refining.
Although the ladle is used as the pre-refining container 50 in FIG. 2, it goes without saying that an induction heating furnace to which a normal reduced pressure atmosphere can be applied may be used instead of the ladle.

[0026]

EXAMPLES The present invention was carried out in the following procedure using the refining apparatus shown in FIG. First, the molten metal from an electric arc furnace (not shown) is transferred to a ladle (not shown), and the molten metal is discharged to the preliminary refining furnace 5 from the bottom of the ladle. At this time, the slag floating on the upper surface of the molten metal in the ladle was stopped before the slag flowed out to the preliminary refining furnace, so that the slag from the electric arc furnace was removed so as not to be brought into the preliminary refining furnace 5. Subsequently, the lid 3 is applied, and the vacuum evacuation system 6a is used to remove the atmosphere shut-off chamber a.
Ar pressure is reduced to reduce the oxygen content in some cases.
The gas is replaced to obtain a predetermined atmosphere. Next, the molten metal is preliminarily refined in the air-shielding chamber a by the induction heating refining furnace 5. In the pre-refining of the present invention, since the refining temperature and time can be selected almost freely by induction heating, it is possible to reliably obtain a molten metal having a predetermined refining level.

In parallel, in the atmosphere shut-off chamber b, a re-refining vessel is set. In some cases, in an Ar gas atmosphere near atmospheric pressure, the gas plasma heating device 23 is used to heat the refractory lining of the re-refining vessel 27 in a molten metal-free state,
Preheat the refractory in the re-refining container by heating it above the tap water temperature.
It is passivated in the absence of pollution such as air and combustion produced gas, and is kept waiting for heating to minimize the contamination of the molten metal when the molten metal is received and the temperature decrease. Atmosphere shut-off chamber a
When the refining in step a is completed, the preheating operation of the re-refining vessel by the inert gas plasma heating device 23 in the atmosphere shut-off chamber b is stopped, and at the same time, the atmosphere shut-off chamber b is exhausted by the vacuum exhaust system 6b. With the same pressure, the sluice valve 4 is opened, and the re-refining vessel 27 is moved to the position 27 'of the atmosphere shut-off chamber a by the rail and the carriage 24 through the opening.

By tilting the induction heating refining furnace 5,
The molten metal is poured into the re-refining container 27. After that, the rail and the carriage 24 promptly move the re-refining container holding the molten metal to the re-refining position (27), and then adjust the atmosphere in the atmosphere shut-off chamber b to an atmosphere in which the inert gas plasma can be applied.
A slag forming agent containing aO as a main component is added and heated by a gas plasma heating device to melt and heat the slag forming agent. At this time, the Ar gas is blown from the porous plug 28 through the inert gas introduction system 7b and stirred, so that the impurities that have flowed into the re-refining container 27 together with the molten metal by tapping due to tilting and the like are converted into a fresh and active slag forming agent. Absorb. In this embodiment, Ar gas blowing and stirring were performed, but this means is not essential in the present invention and can be arbitrarily adopted. After the gas plasma shown in Table 1 is heated, the re-refining is finished, and after a predetermined sedation, it is cast into the ingot case 30 through the sliding nozzle 29.

The results of refining high Ni stainless steel (JIS SUS304) by the above-mentioned procedure are shown.
The conditions for refining are shown in Table 1. In addition, Table 1
The adjustment of the carry-in amount of the slag from the electric arc furnace to the pre-refining shown in Fig. 1 is carried out at a time when the molten metal from the electric arc furnace is introduced from the ladle into the vacuum induction heating furnace 5 by 0.1 wt. %, Adjusted to 0.4 wt%, adjusted to 0.8 wt%.

[0030]

[Table 1]

Pre-refining and re-refining were carried out under the conditions shown in Table 1. In addition, during the transition period from the pre-refining to the re-refining, the atmosphere of the pre-refining was kept in order to prevent the molten metal from being oxidized by the contact with the atmosphere. At this time, the progress of refining is shown in FIG. 3, FIG. 4 and FIG.
Shown in As is clear from comparing FIGS. 3 to 5, the introduction of slag significantly suppresses the reduction of oxygen and carbon in the pre-smelting. It should be noted that, in FIG. 5, remelting was not performed because the melting loss of the furnace wall of the preliminary refining furnace became large and the refining was stopped midway. Also, in the subsequent re-refining, it can be seen that it is difficult to reduce the oxygen content in No. 2 which has a large amount of slag brought in, compared to No. 1 which has substantially no slag brought in. This indicates that the slag from the primary refining melting furnace also hinders the refining effect during re-smelting, and in order to obtain a highly clean alloy, remove the slag from the primary refining furnace as much as possible. It turns out that it is necessary. Further, as shown in FIGS. 3 and 4, in the pre-refining, it is particularly effective in reducing carbon and oxygen, and in the re-refining in which a slag-forming agent is added, it is effective in reducing oxygen and sulfur. Therefore, it can be seen that the combination of pre-refining and re-refining is effective for obtaining an alloy with high cleanliness.

[0032]

As described above, according to the present invention, the molten steel obtained in the primary melting and refining furnace is subjected to slag removal, so that the molten metal is not covered with slag, so that the reduced pressure atmosphere is maintained. Since it comes into direct contact with the molten metal, the pre-refining in which a reduced pressure atmosphere is applied can be rapidly advanced, and the amount of impurities in the pre-refining can be further reduced. Furthermore, according to the present invention, re-refining in which a slag-forming agent is added to impurities or inclusions that cannot be completely reduced in the pre-refining is applied, so that the impurities can be reduced to the limit. Therefore, the presence of minute impurities or inclusions is industrially useful for obtaining a high-quality alloy that affects physical properties, chemical properties, mechanical properties, electrical properties, or magnetic properties.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the configuration of a refining apparatus used when performing pre-refining and re-refining in separate containers in the manufacturing method of the present invention.

FIG. 2 is a diagram showing an example of a configuration of a refining apparatus used when performing pre-refining and re-refining in the same container in the manufacturing method of the present invention.

FIG. 3 is a diagram showing the transition of impurity elements in the refining process in the case where slag is not substantially brought in from the primary melting / smelting furnace.

[Fig. 4] No slag brought in from the primary melting / smelting furnace.
It is the figure which showed the transition of the impurity element in the refining process in case of 4 wt%.

[Fig. 5] Carrying in slag from the primary melting / smelting furnace is 0.
It is the figure which showed the transition of the impurity element in the refining process in case of 8 wt%.

[Explanation of symbols]

1 pre-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 feeding system, 20 re-refining device, 22 partition door, 23 gas plasma heating device, 24 rail and carriage, 27 re-refining container (re-refining position), 28 porous plug, 29 sliding nozzle, 30 ingot case, 31 trolley, a atmosphere shut-off chamber, b atmosphere shut-off chamber, 50 container, 51 molten metal, 5
2 induction heating coil, 53 container vacuum lid, 54 vacuum exhaust system 55 charging device, 56 plasma heating torch

Claims (4)

[Claims] 1. The molten steel obtained in the primary melting and refining furnace is slag removed, then heated by a container having a heating means and pre-refined under an atmosphere of reduced pressure and oxygen of 300 ppm or less, and then the molten metal is obtained. A refining method for molten metal, which comprises adding a slag-making agent and heating the mixture by gas plasma to re-refining. 2. The method for refining a molten metal according to claim 1, wherein an atmosphere of oxygen of 300 ppm or less is maintained during a transition period from preliminary refining to re-refining. 3. The method for refining a molten metal according to claim 1, wherein the heating in the pre-refining is induction heating. 4. The molten metal refining method according to claim 1, wherein the refining is performed in a container different from the preliminary refining.