JP2003171714A - Molten steel refining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vacuum refining method of molten steel for making inclusions finer by preventing the inclusions in the steel from becoming coarsen in a manufacturing method of steel of high cleanliness using a secondary refining device and a normal pressure secondary refining device to provide the agitating force by blowing gas into the molten steel by decompressing the surface of the molten steel in a vacuum refining device, in particular, a ladle. <P>SOLUTION: In a molten steel refining method using a vacuum and decompression refining device, a MgO source is added to the molten steel. Slag is formed on the surface of the molten steel under decompression. The MgO concentration in the slag is set to be ≥5% to ≤20%. The vacuum and decompression refining device can realize a decompressed atmosphere on the surface of the molten steel in the ladle, and the molten steel is agitated by blowing gas into the ladle. In the vacuum and decompression refining device, a vacuum and decompression suction tank is directly connected to an upper part of the ladle to evacuate the atmosphere in the ladle. <P>COPYRIGHT: (C)2003,JPO

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 molten steel capable of refining inclusions in steel, and more particularly to a refining method using a vacuum / vacuum refining apparatus.

[0002]

2. Description of the Related Art Recently, the quality required for steel materials has become increasingly strict and diversified, and the development of technology for producing cleaner steel is strongly desired. Oxide inclusions in steel materials, in particular alumina (Al ₂ O ₃ ) inclusions, are the causes of wire breakage of wire materials such as tire cords, the cause of deterioration of rolling fatigue characteristics in bar steels such as bearing steels, and the cause of DI cans and the like. It is known that thin steel sheets cause cracking during can-making, and there has been a strong demand for reduction thereof.

Regarding the reduction / removal of alumina-based inclusions, deaeration, slag reforming, etc. are focused on the reduction of deoxidation products by the application of secondary refining equipment such as RH vacuum degassing equipment and powder blowing equipment. In order to reduce the inclusions, we have combined the prevention of reoxidation by slag and the reduction of inclusions of oxide-based inclusions by slag cutting.

In Japanese Unexamined Patent Publication (Kokai) No. 5-31225, Al ₂ O ₃ is added by adding metallic Mg to molten steel.
It is described that the coarsening due to the agglomeration of Al ₂ O ₃ is prevented and Al ₂ O ₃ is dispersed in the steel material in a small size of 2 μm or less to avoid the adverse effect of Al ₂ O ₃ on the steel material quality. With Al ₂ O ₃ · MgO and Al ₂ O ₃ is reformed, and that it is possible to completely prevent the aggregation and coarsening of Al ₂ O _3.

However, the method of adding metallic Mg to the molten steel has the following problems. Since the temperature of the molten steel to which Mg is added is equal to or higher than the boiling point of metal Mg (1107 ° C.), Mg easily evaporates. Even if Si, Al,
Even if it is added by mixing with Fe—Si or the like, it is a mixture and the activity of Mg remains 1. Therefore, its evaporation behavior is the same as that of adding metallic Mg alone. Al-M
When alloyed such as g or Si-Mg, the evaporation loss at the time of addition can be reduced to some extent, but after dissolution in molten steel, evaporation cannot be suppressed as in the case of addition with metallic Mg. Addition yield is 50% instead of expensive elements
It is as low as or less and costly. Ordinary Fe-S
i, Fe-Cr and other ferroalloys for adjusting the composition of molten steel do not evaporate, so it can be added during vacuum refining such as RH, but when Mg is added in a vacuum, it is sharper than at normal pressure. Therefore, it is necessary to separately provide a step of adding at normal pressure after vacuum refining as the time of addition. Therefore, it is necessary to raise the temperature of the molten steel in the previous step in consideration of the temperature loss during the addition, which also contributes to the cost increase. Furthermore, since it is an active metal, it is necessary to store it in a strictly controlled state, which is also a problem. In Japanese Patent No. 3033000, when refractory is applied to molten steel in the production of high-carbon chromium bearing steel, refractory in a decompression container is treated with MgO:
40% or more magnesia system, vacuum degree during processing 5T
A method is described in which the processing time is set to or or less and a predetermined processing time is secured. According to the publication, Mg in MgO refractory
Since O is in contact with high carbon molten steel under reduced pressure, Mg and O
Was decomposed into Mg and the generated Mg was Al ₂ generated in the molten steel.
Converts O ₃ into Al ₂ O ₃ .MgO or MgO. The generated Al ₂ O ₃ .MgO or MgO is Al
_{It is} said that the interface energy with molten steel is smaller than that of ₂ O ₃ and the size becomes finer.

As a secondary refining apparatus for molten steel, a vacuum refining apparatus such as RH and DH, an arc heating slag refining apparatus typified by LF, etc. are generally used for the purpose of degassing and removing inclusions of molten steel. In the production of high-cleanliness steel such as bearing steel, a process in which LF and RH degassing are used together is generally performed as necessary. On the other hand, as a method of directly depressurizing the molten steel surface in the ladle and efficiently performing the reaction between the slag and the molten steel under vacuum, the VOD method, VAD method, SS
-The VOD method and the like have been developed. In these methods, as a means for directly depressurizing the molten steel surface in the ladle, the ladle is housed in a decompression container capable of housing the entire ladle to decompress the entire ladle. On the other hand, Japanese Patent Laid-Open No. 2000-178
As disclosed in Japanese Patent No. 637, there is a method in which a ladle itself is used as a lower decompression tank, and an upper decompression tank is brought into close contact with the upper portion of the ladle to decompress the molten steel surface in the ladle. A slag is formed on the surface of the molten steel in the ladle that has been placed under reduced pressure, and the molten steel is agitated by blowing gas from the ladle bottom or the immersion lance. RH
Compared to the vacuum degassing method, the stirring power of the molten steel in the ladle is sufficiently high so that the molten steel can be modified with slag, and oxide inclusions in the molten steel are moved and removed into the slag to achieve high cleanliness. Steel can be produced.

In the production of high cleanliness steel in which oxides are removed by the slag formed on the surface of molten steel in the ladle, the FeO and MnO concentrations are lowered as the slag composition, and CaO /
It is necessary to make the composition high in SiO ₂ . CaO / SiO ₂
As a result, the SiO ₂ concentration in the slag decreases. As a result, Al reoxidation in the molten steel due to SiO ₂ is suppressed, and the oxygen content of the molten steel can be lowered (TO can be reduced).

[0008]

When the CaO / SiO ₂ content of the slag is increased to lower the oxygen content of the molten steel as described above, the activity of CaO in the slag is high, and the equilibrium reaction between the slag and the metal causes Input of Ca occurs from slag to metal. In particular, in the secondary refining method in which the molten steel surface in the ladle is decompressed and the stirring force is imparted by blowing gas into the molten steel, the influence of Ca input becomes particularly large because the mixing of slag and metal is large. When the composition of the initial inclusions in the secondary refining is Al ₂ O ₃ , the input Ca and Al ₂ O ₃ react with each other to form CaO—Al ₂ O ₃ type inclusions, and the inclusions become coarse. It will be.

In the method described in the above-mentioned Japanese Patent No. 3033000, in which the refractory in the depressurization vessel is made of a magnesia system of MgO: 40% or more, a treatment time longer than a predetermined time is required. By performing the treatment for this treatment time, the Al ₂ O ₃ inclusions in the molten steel become MgO.Al ₂ O ₃ or MgO.
The composition changes and becomes finer. However, in the secondary refining, as described above, Ca input from slag with high CaO / SiO ₂ also occurs at the same time, so when the treatment time is long, Al ₂ O ₃ is reformed to MgO · Al ₂ O ₃ or MgO. However, CaO—Al ₂ O ₃ -based inclusions are also likely to be formed, which may cause coarsening of the inclusions, resulting in unstable composition control of inclusions. Especially, in the secondary refining method in which the molten steel surface in the ladle is decompressed and the stirring force is imparted by blowing gas into the molten steel, the Ca input is larger than that of RH vacuum degassing because the mixing of slag and metal is large. It is very large, and the effect of refining inclusions by Mg from the MgO refractory cannot be exerted. Further, although the one described in the above-mentioned Japanese Patent No. 3033000 is intended for high carbon steel, it is required to manufacture highly clean steel by refining inclusions even in steel having low carbon.

The present invention relates to a vacuum refining apparatus, in particular, a secondary refining apparatus for depressurizing the surface of molten steel in a ladle and imparting a stirring force by blowing gas into the molten steel, and a highly clean using a normal pressure secondary refining apparatus. An object of the present invention is to provide a method for refining molten steel capable of preventing the inclusions from coarsening and refining the inclusions in a method for producing a grade steel.

[0011]

[Means for Solving the Problems] The oxide MgO is
Since the vapor pressure is not high like metal Mg or Mg alloy, it can be added with a good yield even if it is added in vacuum. In addition, MgO has a property of easily dissociating into Mg and O at a high temperature such as a molten steel temperature. Especially under reduced pressure, the tendency becomes large. If a MgO source is added to the molten steel after deoxidizing with a deoxidizing agent containing Al, or the MgO concentration of the slag formed on the molten steel surface is increased, Mg at the interface with the molten steel
O dissociation occurs and the resulting O reacts with Al in the molten steel Al ₂ O _3. On the other hand the resulting Mg is Al ₂ O ₃ in the molten steel
Reacts with MgO.Al ₂ O ₃ or MgO to form. Since the inclusion composition rapidly changes from Al ₂ O ₃ to MgO.Al ₂ O ₃ , this deterioration precedes the Ca input from the slag, and this composition is a stable composition even if Ca input occurs. , It is hard to change to harmful CaO / Al ₂ O ₃ inclusions. In addition, MgO.Al ₂ O ₃ has a small aggregation / coalescence property and can keep inclusions fine. Therefore, by adding MgO to molten steel during molten steel refining,
Inclusions in molten steel can be refined at low cost.

The present invention was made based on the above findings, and the gist of the invention is as follows. (1) A method for refining molten steel using a vacuum / reduced pressure refining apparatus, wherein a MgO source is added to the molten steel. (2) The method for refining molten steel according to (1) above, wherein slag is formed on the surface of the molten steel under reduced pressure. (3) The refining method for molten steel according to the above (2), wherein the MgO concentration in the slag is set to 5% or more and 20% or less. (4) The vacuum / decompression refining apparatus can make the molten steel surface in the ladle a depressurized atmosphere, and blows gas into the ladle to stir the molten steel. ) A method for refining molten steel according to any one of (1) to (4) above. (5) The method for refining molten steel according to (4), wherein the vacuum / decompression refining apparatus decompresses the atmosphere in the ladle by directly connecting a vacuum / decompression suction tank to the top of the ladle. (6) A method of refining molten steel at normal pressure using a secondary refining vessel, which comprises adding an MgO source to the molten steel. (7) The method for refining molten steel according to (6) above, wherein slag is formed on the surface of the molten steel and the MgO concentration in the slag is set to 5% or more and 20% or less.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The dissociation of MgO into Mg and O is
It occurs both when the MgO is in the molten steel and in the slag. MgO added to the molten steel causes a dissociation reaction in the molten steel, floats in the molten steel and is taken into the slag on the surface of the molten steel, and then causes a dissociation reaction also in the slag. When incorporated into the slag immediately after the addition of MgO, the reaction proceeds in the slag.

MgO in the molten steel or slag is dissociated by the following equation (1). The generated O is the following (2)
According to the formula, it reacts with Al in molten steel to form Al ₂ O ₃ . On the other hand, the generated Mg is Al which is an inclusion in the molten steel.
₂ O ₃ the following equation (3) or between (4) undergoes a reaction formula, a MgO · Al ₂ O ₃ or MgO. MgO = Mg + O (1) 2 · Al + 3 · O = Al ₂ O ₃ (2) Mg + 4/3 · Al ₂ O ₃ = MgO · Al ₂ O ₃ + 2/3 · Al (3) Mg +1/3 ・ Al ₂ O ₃ = MgO + 2/3 ・ Al (4)

In the molten steel refining in which slag is generated on the surface of the molten steel in the ladle and gas is blown into the molten steel in the ladle to stir,
Since the slag-metal interface is vigorously stirred, the above reaction rapidly progresses between MgO in the slag and the molten steel. When injecting MgO together with the gas blown into the molten steel, the injected MgO directly reacts with Al ₂ O ₃ in the molten steel in the process of floating. Since the reaction interfacial area between the powder MgO and the molten steel is large, a high reaction rate can be realized.

In the above, if the molten steel surface in the ladle is depressurized, M in the slag formed on the molten steel surface under depressurization
Since gO contributes to the reaction and MgO dissociation in the above formula (1) proceeds more actively, the effect of modifying the composition of inclusions can be further enhanced.

RH RH vacuum degassing refines MgO to RH
When added in the vacuum tank, MgO is taken into the molten steel in the vacuum tank, discharged from the vacuum tank together with the molten steel, and finally taken into the slag on the upper surface of the ladle. Therefore, in this case, when MgO is mainly contained in the molten steel, the reaction proceeds particularly under reduced pressure, but MgO taken into the slag also contributes to the reaction.

In the invention in which the refractory in the decompression container is made of magnesia, MgO in the refractory and Al ₂ O _{3 in the} molten steel are used.
The reaction with and takes a certain amount of time, and if the processing time is long, Ca input from the slag occurs during that time and harmful Ca
Occurrence of O-Al ₂ O ₃ -based inclusions and coarsening of the inclusions caused unstable control of inclusions. On the other hand, in the present invention, since the reforming of Al ₂ O ₃ with MgO proceeds rapidly, it is possible to prevent the generation of harmful inclusions due to Ca input. Furthermore, MgO in the slag
Since the activity of CaO can be reduced by increasing the concentration, it also exerts the effect of suppressing the input of Ca itself from the slag to the molten steel.

The MgO concentration in the slag is 5% or more and 20% or more.
The following is preferable. If the MgO concentration in the slag is less than 5%, the effect of modifying inclusions of the present invention by MgO cannot be sufficiently exhibited. Further, if the MgO concentration in the slag exceeds 20%, the fluidity of the slag deteriorates, and the high cleaning effect of the slag cannot be sufficiently exerted.

In the present invention, CaO / Si in slag is used.
It is preferable to set the O ₂ ratio to 3 or more. As a result, the SiO ₂ concentration in the slag decreases, so that the reaction between SiO _{2 in} the slag and Al in the molten steel can be suppressed, and the total oxygen concentration in the molten steel can be reduced. On the other hand, since doing inclusions modification by MgO, it can be high CaO / SiO ₂ concentration to prevent the generation of CaO-Al ₂ O ₃ inclusions of low melting point. Also, since the MgO concentration in the slag is high,
As described above, the Ca input itself can be suppressed by lowering the activity of CaO. On the other hand, Ca in slag
If the O / SiO ₂ ratio is too high, the fluidity of the slag deteriorates, so the upper limit is preferably around 10.

In the present invention in which slag is formed on the surface of molten steel in the ladle and inclusions are controlled by MgO in the slag, as described above, refining is performed by setting the depressurized atmosphere on the surface of the molten steel. The effect can be further exerted. Further, by blowing gas into the ladle and refining the molten steel, it is possible to sufficiently stir the molten steel and the slag when the blown gas floats and separates from the molten steel to accelerate the reaction. By blowing MgO powder together with the blowing gas, the reaction can be further promoted. The atmospheric pressure on the surface of the molten steel in the ladle is preferably 20 Torr or less. It is even more preferable to set it to 10 Torr or less. At 20 Torr or less, Al ₂ O ₃ begins to be converted to MgO and becomes finer than MgO · Al ₂ O ₃ . 10
The conversion rate up to MgO becomes significant below Torr.

As a vacuum / reduced pressure refining device for making the molten steel surface in the ladle a reduced pressure atmosphere, VOD method, VAD method, S
The S-VOD method or the like can be adopted. By sufficiently lowering the atmospheric pressure on the surface of the molten steel in the ladle to a high vacuum, degassing of the molten steel, such as dehydrogenation and denitrification, can be performed simultaneously.

Further, as a vacuum refining device for making the molten steel surface in the ladle a depressurized atmosphere, a vacuum suction / decompression tank 2 as shown in FIG. A vacuum refining device for decompressing the atmosphere can be used. It is not necessary to prepare a vacuum container for accommodating the entire ladle as in the VOD method. Furthermore, since the ladle 1 itself is used as the lower decompression tank, vacuum suction as the upper decompression tank
Since the decompression tank 2 only needs to be prepared, the equipment cost can be significantly reduced and the equipment can be installed in a small space. Vacuum suction / vacuum suction 13 is performed from the upper part of the decompression tank 2 to decompress the atmosphere in the ladle, and Ar gas is blown as the blowing gas 11 from the gas blowing plug 3 provided at the bottom of the ladle 1 to rise in the molten steel. It is possible to stir the molten steel 4 and the slag 5 with the bubbles 6 and simultaneously degas the molten steel.

In the method of directly connecting the vacuum / reduced pressure suction tank to the upper part of the ladle, the vacuum / reduced pressure suction tank is used in order to prevent the splash from splashing in the contact portion between the ladle 1 and the vacuum / reduced pressure suction tank 2. It is preferable that the inner diameter of the body portion of 2 is made smaller than the inner diameter of the upper end portion of the ladle to be equal to or larger than the projected cross-sectional diameter of the rising portion 7 of the molten steel molten metal surface in the ladle generated by the stirring gas blown into the ladle. Further, by providing the cylindrical portion 9 at the lower end of the vacuum / reduced pressure suction tank 2 facing the surface of the molten steel, it is possible to more effectively prevent the splash from splashing to the contact portion 8. A burner 1 for injecting a flame 12 by burning fuel and oxygen gas in a vacuum / decompression suction tank
If 0 is set, the temperature inside the vacuum / reduced pressure suction tank can be always kept high, and it is possible to prevent the molten steel temperature from decreasing during refining.

In the present invention, the MgO source is mainly composed of MgO which is an oxide of Mg, and the material to be added is electromelting magnesia, seawater magnesia, dolomite, M.
MgO-based brick scraps such as gO-C can be used. When the powder is injected into molten steel, the material used is crushed electro-melted magnesia or MgO-based brick waste, and the particle size is preferably 3 mm or less. Since dolomite has a CaO-MgO composition, A in molten steel
It reacts with l ₂ O ₃ to form coarse Al ₂ O ₃ —CaO—MgO-based inclusions. Therefore, when injecting into molten steel, a MgO source not containing CaO is preferable. In addition, when adding molten steel from above in the RH vacuum tank, or when adding molten steel from above in the ladle, a material obtained by crushing electro-melted magnesia or MgO-based brick waste is used as the additive material. The size may be 10 to 50 mm. The smaller the size, the better the reactivity, but if it is too small, the scattering loss is large, so 10 to 50 mm is preferable. When added to the slag, dolomite containing CaO may be used.

The timing of adding MgO in the present invention is as follows.
It is preferable that the molten steel is sufficiently deoxidized with Al or the like. Usually, when tapping a ladle from a primary refining vessel such as a converter, Mn, Si, and Al are added to the ladle to perform primary deoxidation, and subsequently various secondary refining is performed. It is advisable to add MgO.

As a steel type to which the present invention is applied, low-acid steel is used.
It can target all steel grades that require
It For varieties that undergo normal deoxidation of Al, Si and Mn
Al is the main inclusion remaining in the molten steel.₂O₃And the book
With the addition of MgO of the invention, Al ₂O₃Can modify inclusions
You can By slag-metal reaction in secondary refining of molten steel
In the refining of molten steel, which seeks to reduce oxygen in molten steel.
The addition of MgO according to the invention is particularly effective. Low oxygen in molten steel
CaO / SiO for slag₂Even if the ratio is increased,
It is possible to suppress the increase of Ca input into molten steel,
The input Ca is Al₂O₃Coarse inclusions
This is because it can be suppressed.

Further, the grades to which the present invention is applied are not limited to high carbon steels, and can be applied to steels of any carbon content to exert their effects. Low carbon Al-Si killed steel, Al
It has been confirmed that in killed steel, medium carbon Al-Si killed steel, and high carbon steel, Al ₂ O ₃ inclusions are transformed into MgO · Al ₂ O ₃ inclusions.

[0029]

(Example 1) A ladle of 300t bearing steel having a molten steel composition shown in Table 1 is used by using a vacuum / decompression refining device in which a vacuum / decompression suction tank is directly connected to the upper part of the ladle as shown in FIG. The present invention was applied in performing refining. The molten steel subjected to the primary refining in the converter was subjected to Al-Si-Mn deoxidation at the time of tapping, and then subjected to ladle refining. The molten steel 4 and the slag 5 in the ladle 1 are agitated by blowing Ar gas from the gas blowing plug 3 provided at the bottom of the ladle 1. The degree of vacuum in the atmosphere above the ladle was 10 Torr. The slag 5 on the surface of molten steel is
It was formed by adding agglomerate lime from above. M
Electromelted magnesia was used as a gO source, and was added to molten steel and slag from an addition hopper (not shown) provided at the top of the vacuum / vacuum suction tank 2. The processing time for ladle refining was 20 minutes.

The amount of the MgO source added was 4 as shown in Table 2.
The level was set. Except for the addition amount of MgO source, the addition amount of slag forming agent such as CaO for forming slag is the same at each level, and the slag amount before addition of MgO source is about 10 kg / t.
Met. Table 3 shows the slag composition after each level of ladle refining treatment.

Regarding the particle size of inclusions after the refining treatment, the maximum particle size of inclusions in each visual field was measured with an optical microscope for an inspection reference area of 100 mm ² and 30 visual fields in a sample collected from a rolled steel material. Then, it was evaluated as the estimated maximum inclusion particle size in the measurement evaluation area of 30,000 mm ² by the extreme value statistical method. FIG. 2 and Table 4 show the total oxygen analysis results and inclusion particle size measurement results for each sample. When MgO is not added (level 1), M in slag
The gO concentration is 5% or less, and the inclusion composition is 10μ.
15-40 μm with spinel and alumina inclusions of m or less
m calcium aluminate-based inclusions were found, and particularly large inclusions were calcium aluminate-based inclusions. On the other hand, in the samples (levels 2 and 3) to which MgO was added, the particle size of inclusions was as small as 5 to 10 μm, and the composition was Mg.
It was mainly composed of O.Al ₂ O ₃ spinel. Further, in Level 4 in which the MgO source was added at 2.1 kg / t, the total oxygen after treatment was 11 ppm, which is higher than other levels, and this is because the MgO excess addition causes the melting point of the slag to rise above the molten steel temperature. It is considered that the ability to trap inclusions in the slag decreased due to lack of fluidity.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

(Example 2) In refining 300t of high carbon chromium bearing steel in RH degassing refining equipment,
The present invention has been applied. Molten steel that has undergone primary refining in the converter is
After deoxidizing Al-Si-Mn during tapping, RH degassing refining was performed. RH degassing treatment conditions are vacuum degree 0.5-
3 torr, reflux rate 100 ton / min, processing time 20
And the refractory composition in the RH vacuum chamber is MgO: 55%-
_{Cr 2 O 3: 38% -Al} 2 O 3: 7%.

Immediately after starting the treatment, MgO-C was used as a MgO source.
Brick waste was added to the molten steel from the hopper above the vacuum chamber.
Table 5 shows the MgO addition level, and Table 6 shows the slag composition on the molten steel surface in the ladle after the refining treatment. The amount of slag after the treatment was about 10 kg / t. After refining, 65φ material was manufactured by continuous casting and bar rolling.

A sample for inclusion evaluation was taken from this material, and the size and composition of inclusions existing in the evaluation range of an area of 10 mm ² were investigated. The results are shown in Table 7. In Examples 1 and 2 of the present invention, the inclusions were MgO.Al ₂ O ₃ and MgO.
The modified coarse CaO-Al ₂ O ₃ based inclusions was observed. The size of inclusions was 0.3 to 5.2 μm, which was fine. Comparative Example 1 is a case where the MgO source was not added, but the Mg input from the refractory was insufficient in terms of time, so it was not modified and remained Al ₂ O ₃ . Coarse CaO-Al ₂ O ₃ -based inclusions were present, and their size was as large as 2-14.8 μm. Also, Mg
In Comparative Example 2 in which the O source was added in a large amount of 2.1 kg / t, most of the inclusions were modified to MgO.Al ₂ O ₃ , but the addition of a large amount of the MgO source caused the melting point of the slag to be molten steel. When the temperature exceeds the temperature, the ability of the slag to trap inclusions decreases due to insufficient fluidity of the slag. As a result, large inclusions remain and the T. O was higher than other levels, and the cleanliness deteriorated.

[0039]

[Table 5]

[0040]

[Table 6]

[0041]

[Table 7]

(Example 3) In refining 300t of high carbon chromium bearing steel in a ladle type refining equipment under normal pressure,
The present invention has been applied. Molten steel that has undergone primary refining in the converter is
After deoxidizing Al-Si-Mn at the time of tapping, ladle refining was performed. A slag of 10 kg / t was formed on the surface of the molten steel in the ladle, and the molten steel and the slag were stirred by blowing Ar gas using a dipping lance. As the slag forming agent for forming slag and the MgO source of the present invention, the level of addition from the top of the ladle to the surface of the molten steel and the level of blowing from the immersion lance into the molten steel together with Ar gas were adopted. The processing time for ladle refining is 20 minutes, and the ladle refractory is made of Al ₂ O ₃ .

Immediately after the start of the ladle refining process, fused magnesia was added as a MgO source. Table 8 shows the levels of addition of MgO.
Table 9 shows the slag composition after the refining treatment.

After refining, continuous casting, bar steel rolling to 65φ
Manufactured material. Samples for inclusion evaluation were taken from this material, and the size and composition of inclusions existing in 10 mm ² were investigated. T. Table 10 shows O analysis values and inclusion investigation results.
Shown in.

Inventive Example 1 is one in which a MgO source is added to the top of the slag. The inclusions were modified to MgO.Al ₂ O ₃ and no coarse CaO—Al ₂ O ₃ inclusions were found. The size of inclusions was as fine as 1.2 to 8.5 μm.

Example 2 of the present invention is one in which Ar gas was blown into molten steel as a carrier gas by using a MgO source using a dipping lance. As in Invention Example 1, inclusions were MgO.Al ₂
The O ₃ modified, coarse CaO-Al ₂ O ₃ based inclusions was observed. The size of inclusions is 1.5 to 7.3μ.
It was as fine as m.

In Comparative Example 1, no MgO source was added, but the inclusions were not modified and remained Al ₂ O ₃ . In addition, there are coarse CaO-Al ₂ O ₃ inclusions,
It was as large as 1.8 to 18.2 μm.

In Comparative Examples 2 and 3, the MgO source was 2 kg /
In this case, a large amount of t was added. Most inclusions are M
Although it has been modified to gO.Al ₂ O ₃ , the slag melting point becomes higher than the molten steel temperature due to the addition of a large amount of MgO source, and the ability to trap inclusions in the slag is reduced due to insufficient fluidity of the slag. While the inclusion remains, T. O was higher than other levels, and the cleanliness deteriorated.

[0049]

[Table 8]

[0050]

[Table 9]

[0051]

[Table 10]

[0052]

According to the present invention, by adding MgO to molten steel in the secondary refining, Mg produced by dissociation of MgO reacts with Al ₂ O ₃ in the molten steel and MgO.
As Al ₂ O ₃ and MgO are generated, because the composition is a stable composition occurs is Ca inputs, harmful CaO · Al ₂
Hard to change to O ₃ type inclusions. Also, MgO / Al ₂ O ₃
Has less aggregation / coalescence, and can keep inclusions fine. Therefore, the slag composition of molten steel refining should be high CaO / Si.
It becomes possible to use O _2, and it is possible to promote the reduction of oxygen in molten steel.

By adding MgO in vacuum / vacuum refining, dissociation of MgO can be promoted and the effect of adding MgO can be further enhanced.

By adding MgO in molten steel refining in which slag-metal refining is performed by stirring molten steel and slag, modification of inclusions by MgO can be promoted.

[Brief description of drawings]

FIG. 1 is a schematic view showing a vacuum / reduced pressure refining device in which a vacuum / reduced pressure suction tank is directly connected to an upper part of a ladle.

FIG. 2 is a diagram showing an inclusion evaluation result by an extreme value statistical method.

[Explanation of symbols]

1 ladle 2 vacuum / vacuum suction tank 3 gas injection plug 4 Molten steel 5 slag 6 bubbles 7 Rising part 8 Adhesion part 9 Cylindrical part 10 burners 11 blown gas 12 flames 13 Vacuum suction

Claims (7)

[Claims] 1. A method for refining molten steel using a vacuum / vacuum refining apparatus, wherein a MgO source is added to the molten steel. 2. The method for refining molten steel according to claim 1, wherein slag is formed on the surface of the molten steel under reduced pressure. 3. The MgO concentration in the slag is 5% or more and 20
% Or less, The method for refining molten steel according to claim 2, wherein 4. The vacuum / vacuum refining apparatus can make the molten steel surface in the ladle a depressurized atmosphere, and a gas is blown into the ladle to stir the molten steel.
4. The method for refining molten steel according to any one of 1 to 3. 5. The method for refining molten steel according to claim 4, wherein the vacuum / decompression refining apparatus decompresses the atmosphere in the ladle by directly connecting a vacuum / decompression suction tank to the upper part of the ladle. . 6. A method for refining molten steel using a secondary refining vessel at a normal pressure, wherein a MgO source is added to the molten steel. 7. The method for refining molten steel according to claim 6, wherein slag is formed on the surface of the molten steel and the MgO concentration in the slag is set to 5% or more and 20% or less.