JPH0565525A - Ultra low carbon steel melting method - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a decarburizing method for decarburizing molten steel, which can easily refine steel without much difference from the decarburizing time by a conventional vacuum degassing apparatus. [Composition] Injecting an inert gas from the bottom of the ladle or the auxiliary lance, pushing the oxidizing slag on the molten steel surface toward the ladle wall, and from the top using a lance with a refractory disk at the tip. While blowing the inert gas, the lance is lowered to near the surface of the molten steel to form a flow of the inert gas between the refractory disc and the surface of the molten steel, and the oxygen concentration in the molten steel is adjusted to 25
Stable decarburization to the extremely low carbon region while controlling the range of 0 to 700 ppm. [Effect] Maintenance is easy, and ultra-low carbon steel can be melted reliably and inexpensively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting ultra low carbon steel.

[0002]

2. Description of the Related Art Among steel sheets for thin plates to be pressed, such as automobile steel sheets, there is known an ultra-low carbon steel in which the carbon concentration in the steel sheet is reduced as much as possible in order to improve the workability. Conventionally, in order to produce this ultra-low carbon steel, after the molten steel that has been decarburized to a carbon concentration of about 0.04% in a converter or the like is received in a container such as a ladle in an undeoxidized state. , RH method and DH method using a process having an exhaust device such as a vacuum degassing device, a part of the molten steel is placed in a reduced pressure (vacuum) atmosphere, and the pressure on the gas side is reduced to reduce the interface between the gas and the molten steel. CO
Under conditions of reducing gas partial pressure, decarburization by the method of reacting carbon and oxygen in molten steel shown in the following formula, and then adding alloy to adjust to the target molten steel composition and adjusting Is widely practiced.

C + O → CO (1) Generally, when decarburizing molten steel, as shown in the equation (1),
A method is used in which carbon in molten steel is reacted with oxygen to generate CO gas, and the CO gas is removed to the gas side. Oxygen is required in the molten steel to promote this decarburization reaction, so for molten steel with a high carbon concentration,
Generally, a pure oxygen converter system is used in which oxygen is simultaneously supplied from above or below, or from above and below. However, in this method, when the carbon concentration in the molten steel becomes about 0.04% or less, the progress of decarburization stagnates, rather the oxidation of iron occurs, the yield of molten steel decreases, and the concentration of manganese, which is an effective component in molten steel, decreases. There are problems such as lowering.

Therefore, even if the carbon concentration is lower than this, iron is not oxidized and carbon in the molten steel is preferentially oxidized so that a gaseous or solid oxidation source is supplied to increase the oxygen concentration in the molten steel. The vacuum degassing apparatus is widely used as a method of advancing the reaction of the formula (1) in the right direction by lowering the partial pressure of carbon monoxide on the gas side.

That is, in the following formula (2), if the partial pressure P _CO of carbon monoxide on the gas side is made small, the carbon concentration can be made smaller even if the oxygen concentration in the molten steel is the same. Is.

[0006]

[Equation 1]

P _CO : Partial pressure of carbon monoxide on the gas side [C]: Carbon concentration in molten steel [O]: Oxygen concentration in molten steel K: Equilibrium constant of reaction According to the above method, the degree of pressure reduction is increased ( Increase the degree of vacuum)
Since the decarburization reaction to the right as shown in the equation (1) progresses, the carbon concentration of 0.005% can be achieved by maintaining the molten steel surface under a vacuum as high as possible by the RH method or the DH method.
The following extremely low carbon steels can be melted.

Here, in order to further reduce the decarburization rate,
A method of adding an oxidizing gas such as oxygen or carbon dioxide, or a solid-state oxidation source such as iron oxide in a vacuum degassing apparatus (for example, JP-A-49-34414 and JP-A-5-34414).
No. 1-151211, Japanese Patent Laid-Open No. 51-151212), a method of blowing a large amount of gas into molten steel to increase the interfacial area of the reaction (Japanese Patent Laid-Open No. 52-5641), and the like have been developed. ..

[0009]

The ultra-low carbon steel melting method using the above-mentioned vacuum degassing apparatus is a very effective method in terms of equilibrium because it lowers the carbon concentration, and is now widely used. However, as described above, a very large and expensive vacuum device is required to bring a part of molten steel into contact with a high depressurized state. Further, since the molten steel is treated under reduced pressure, melting loss of refractory materials and the like is severe, and in order to maintain the reduced pressure state at high temperature, detailed maintenance is required to improve the adhesion of each fitting portion.

When gas is blown into the molten steel while the molten steel is in contact with the depressurized state, the molten steel scatters into the surrounding closed container when the gas leaves the surface of the molten steel. Not only does solidified steel adhere and the yield of molten steel lowers, but a great deal of labor is required to remove this adhered metal. Furthermore, the device for depressurizing molten steel is used with sufficient preheating, of course, but it is difficult to uniformly preheat the entire device to a certain temperature because the entire device is large. Then, the temperature drop of molten steel is large. Therefore, in order to stably cast the decarburized molten steel by a process such as continuous casting, it is necessary to compensate for the temperature drop during the decarburization treatment, and the processing cost becomes high. was there.

On the other hand, the present inventors have conducted research and development on a method for stably decarburizing carbon concentration in molten steel to 0.005% or less without the need for a conventional large-scale vacuum device. As a result, in order to reduce the partial pressure of carbon monoxide gas at the interface between the molten steel and the gas, an expensive and large-scale vacuum degassing device or the like is used to hold a portion of the surface of the molten steel under reduced pressure for treatment. Even if it is not used, if an inert gas such as argon or nitrogen is blown to the interface to remove the carbon monoxide gas at the interface and reduce the partial pressure thereof, the carbon concentration will be 0.005% or less under atmospheric pressure. It was found that the decarburization reaction proceeded sufficiently until now, and that it was possible to produce ultra low carbon steel.

[0012] First, on the surface of the molten steel having a carbon concentration of 0.04 to 0.02% in which the area occupied by the oxidizing slag on the surface of the molten steel in the ladle is reduced to 20% or less, the oxygen partial pressure is 0.5.
A method for producing an ultra-low carbon steel having a carbon concentration of 0.02% or less, in which a mixed gas of an oxidizing gas and an inert gas adjusted to 0.01 atm is sprayed to decarburize while suppressing the oxidation of the molten steel. (Japanese Patent Application No. 63-153454), and a specific space is formed by immersing a refractory-immersed body in an area where the oxidizing slag on the molten steel surface in the ladle is removed by 20% or more of the total surface area. Space carbon concentration 0.04-0.03%, oxygen concentration 400
Ultra-low carbon with a carbon concentration of 50 ppm or less by supplying an oxidizing source from outside so as to maintain the oxygen concentration in the molten steel within the range of 250 to 700 ppm while spraying an inert gas on the surface of molten steel of up to 700 ppm. A method for melting steel has been proposed (Japanese Patent Application Nos. 3-9730 and 39731).

The above-mentioned invention is an epoch-making one in which ultra-low carbon steel, which could not be melted without using a vacuum degassing apparatus, can be melted with more inexpensive equipment. In order to increase the decarburization rate in the present invention, it is effective to immerse a dipping pipe having a larger diameter in order to secure a free surface area of molten steel without slag. When the temperature of the molten steel existing during the period decreases, solidified steel called metal is generated in this portion, so there is a limitation that the diameter of the immersion pipe cannot be unnecessarily enlarged.

In addition, when there is little adhesion of metal to the inside of the dip tube, but when a large number of treatments are carried out continuously, the metal removal work is performed together with the repair of the refractory material forming the dip tube. It is necessary to extend the time for supplying molten steel to the continuous casting in the next step because of the replacement work of the immersion pipe for this purpose. In view of such problems, the present invention solves these problems, inexpensive equipment, ultra-low carbon steel equivalent to that produced by a conventional vacuum degassing device, stably and It was developed to provide a melting method that can be continuously supplied to the casting process.

[0015]

DISCLOSURE OF THE INVENTION According to the present invention, a refractory material is provided at the tip of a ladle, in which an inert gas is blown from the bottom or auxiliary lance of the ladle and the oxidizing slag on the molten steel surface is pushed toward the wall of the ladle. While spraying an inert gas from the upper part using a lance provided with a disk-making disc, the lance is lowered to near the surface of the molten steel to form a flow of the inert gas between the refractory disk and the surface of the molten steel. The gist of the present invention is a method for melting ultra-low carbon steel, which is characterized in that molten steel is decarburized while controlling the oxygen concentration in the range of 250 to 700 ppm.

[0016]

The present inventors have been able to spray an inert gas onto the surface of molten steel from which slag has been removed, and at the same time control the oxygen concentration in the molten steel, without using a conventional large-scale vacuum degassing device. Clarified that it is possible to melt ultra low carbon steel. Here, increasing the decarburization rate and shortening the treatment time required for decarburization can reduce the cost of compensating for the decrease in the temperature of the molten steel, and when casting the refined molten steel in continuous casting, etc. It can be said that the effect is very large because the casting speed is increased and the productivity is increased.

As a method for increasing the decarburization rate, it can be said that increasing the area of the slag-free molten steel free surface, which is the reaction boundary area where the decarburization reaction proceeds, can be said to be effective. Therefore, the present inventors have eliminated the slag existing on the molten steel surface, as a result of the development of means for always ensuring a molten steel surface without slag, as a result of blowing an inert gas from the bottom of the ladle or the auxiliary lance, With the oxidizing slag on the molten steel surface pushed against the ladle wall, a lance with a refractory disk at the tip was used to blow an inert gas from the top while lowering the lance to near the molten steel surface for fire protection. It was found that by forming an inert gas flow between the product disk and the surface of the molten steel, it is possible to prevent the oxidizing steel slag from staying and coating the surface of the molten steel in the decarburization treatment zone. Furthermore, by securing a molten steel free surface without slag by this method and decarburizing the molten steel while controlling the oxygen concentration of the molten steel in the range of 250 to 700 ppm while spraying an inert gas on this molten steel surface, It was confirmed that decarburization to the extremely low carbon region can be stably carried out in a short time.

In the present invention, the refractory disc at the tip of the lance is lowered to the vicinity of the surface of the molten steel to form a flow of an inert gas between the refractory disc and the surface of the molten steel. The size of the refractory disc is 50% or more of the inner diameter of the ladle (area ratio) because the decarburization treatment area is secured by the flow action and the molten steel surface in this treatment area is prevented from being coated with oxidizing slag. Is preferably 25% or more), and a larger diameter is preferable in order to increase the decarburization rate.

Further, as the material of the refractory disc, since it is not necessary to immerse it in molten steel, it is not necessary to purposely use a high-grade one, and an ordinary alumina-based or magnesia-based refractory is sufficient. Further, the reason for using an inert gas as the gas to be sprayed on the surface of the molten steel is that when the oxygen concentration in the sprayed gas exceeds 5%, the oxidation of iron occurs preferentially over the oxidation of carbon on the surface of the molten steel, This is because molten iron oxide is generated at the interface, and the free surface area of the molten steel from which the slag has been removed is reduced by this oxide, and the decarburization rate is reduced. As the type of inert gas used here, argon gas is generally used, but helium gas can also be used, and if there is no problem with the material of the steel, nitrogen gas can also be used. You may mix and use.

On the other hand, in order to proceed with the decarburization reaction, oxygen which oxidizes carbon is required as shown in the equation (1). Therefore, the concentration of oxygen in the molten steel before decarburization is set to a range of 400 to 700 ppm obtained when blowing is stopped at a carbon concentration of 0.04 to 0.03% in normal converter blowing, Regarding the oxygen that decreases with the progress of decarburization while oxidizing carbon using oxygen in this molten steel, while measuring its concentration from time to time, if the concentration decreases, a new surface is added to the molten steel surface from which the slag has been removed. In order not to reduce oxides and reduce the reaction interface area of decarburization, the concentration should be 250-700p.
It is necessary to control in the range of pm. The oxide newly formed on the surface of the molten steel from which the oxidizing slag has been removed is discharged and removed from the decarburization treatment area by the flow of the inert gas, but the oxidizing steel slag covers the molten steel interface during the process. Therefore, it is preferable to take care not to generate excessively. As a method for adjusting the oxygen concentration in the molten steel, pure oxygen gas, air, carbon dioxide gas, a single gas such as water vapor, or a gas such as a mixed gas of these oxidizing gas and carbon monoxide gas is continuously blown. A simple method is to intermittently mix with the continuous inert gas and supply while spraying. In addition, a method of blowing into the molten steel through the auxiliary immersion lance from the space between the refractory disc and the wall around the ladle, or a solid oxidation source is used to remove the inert gas through the auxiliary immersion lance. It is also possible to adopt a method of blowing using.

It is necessary to control the oxygen concentration to 250 ppm or more because if it is less than this concentration, the decarburization rate tends to decrease, and the reason for controlling to 700 ppm or less is that the decarburization rate is higher than this. This is because the effect of oxygen concentration on the charcoal velocity is no longer observed, and the amount of inclusions generated when the oxygen is subsequently deoxidized increases, which is also unfavorable in terms of quality.

By such a method, in order to secure a reaction interface area for decarburization without slag, it is possible to prevent further generation and accumulation of oxides on the surface of the molten steel from which the oxidizing slag has been removed first, and The decarburization rate can be maintained at a high level. For the above reasons, the carbon concentration should be 0.0
It is needless to say that vigorous stirring of the molten steel is effective so that carbon in the molten steel is always supplied to the reaction interface when it is desired to proceed to as low as possible at 05% or less and within a short time. Yes. In order to stir the molten steel here,
It is preferable to increase the amount of gas supplied from the bottom of the ladle or from the auxiliary refractory lance soaked, and the gas for this purpose is preferably the same inert gas as the gas blown from the upper surface. Further, as the gas for stirring molten steel, pure oxygen gas for controlling the oxygen concentration described above, air,
A single gas such as carbon dioxide gas or water vapor, a mixed gas of these oxidizing gases and carbon monoxide gas, or a mixed gas of these oxidizing gases and an inert gas may be used.

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 to 5.

[0024]

【Example】

Example 1 In a converter, carbon 0.04%, manganese 0.
50 ton of molten steel 1 melted to 30% was tapped into a ladle 2 having an inner diameter of 2.2 m in an undeoxidized state. The composition of the molten steel in the ladle 2 after tapping is 0.037% carbon and 450 ppm oxygen,
The temperature of the molten steel was 1670 ° C.

Thereafter, as shown in FIG. 1, 1 gas of Ar gas was discharged from the refractory porous plug 3 arranged at the bottom of the ladle 2.
It was blown at a flow rate of 0 Nm ³ / hr to secure the state in which the oxidizing slag 4 was pushed to the inner wall of the ladle. Next, an upper blowing lance 5 provided with a refractory disc having a diameter of 1.6 m (area ratio to the ladle cross-sectional area: about 50%) from the top of the molten steel surface to the tip.
The Ar gas was sprayed at a flow rate of 700 Nm ³ / hr through the lance to gradually lower the lance, and the lance was placed on the surface of the molten steel as shown in FIG. Here, the lance 5 was positioned approximately 20 cm above the molten steel level by the blowing gas flow, and a molten steel surface free of the oxidizing slag 4 was secured under the refractory disc. In this state, Ar gas was continuously blown from the bottom of the ladle at a flow rate of 10 Nm ³ / hr, and decarburization was performed for 15 minutes while stirring.

When the oxygen concentration was measured 5 minutes on the way, 29
Since it showed 0 ppm, pure oxygen gas was mixed with Ar gas blown from the upper blowing lance 5 at 30 Nm ³ / hr (oxygen gas concentration 4.0%), and sprayed at 1 minute intervals for 6 minutes to bring the oxygen concentration up to 450 ppm. Raised. The molten steel temperature after this decarburizing treatment was 1635 ° C, the carbon concentration was 0.003%, the manganese concentration was 0.28%, and the carbon concentration was stable at 0.005%.
I have reached the following:

Example 2 In a converter, 50 ton of molten steel 1 prepared by melting hot metal to 0.044% carbon and 0.33% manganese has an inner diameter of 2.2.
Steel was tapped into the ladle 2 of m in an undeoxidized state. The composition of the molten steel in the ladle 2 after tapping was 0.042% carbon and 420 ppm oxygen, and the temperature of the molten steel was 1660 ° C.

Then, as shown in FIG. 1, 10 gas of Ar gas was supplied from the refractory porous plug 3 arranged at the bottom of the ladle.
It was blown at a flow rate of Nm ³ / hr to ensure that the oxidizing slag 4 was pushed to the inner wall of the ladle. Next, Ar gas was flown through an upper blowing lance 5 equipped with a refractory disk having a diameter of 1.2 m (the area ratio of the ladle cross-sectional area is about 30%) from the top of the molten steel surface to 700 Nm ³ / hr. The lance was gradually lowered while spraying with, and placed on the surface of the molten steel as shown in FIG. Here, the blown gas flow ensured that the lance 5 was positioned approximately 20 cm above the surface of the molten steel, and the molten steel surface without the oxidizing slag 4 was secured under the refractory disc. In this state, Ar gas was continuously blown from the bottom of the ladle at a flow rate of 10 Nm ³ / hr, and decarburization was performed for 15 minutes while stirring.

When the oxygen concentration was measured 5 minutes on the way, 2
Since it was 50 ppm, the amount of Ar gas sprayed from the upper spray lance 5 was maintained as it was, the auxiliary immersion lance 6 was immersed as shown in FIG. 3, and Ar oxide 20 Nm ³ / hr was used as a carrier gas to remove iron oxide. Inject 5 kg / min for 5 minutes (total 25 kg of iron oxide), oxygen concentration 400 ppm
Raised to. The molten steel temperature after this decarburization treatment is 1620
At ℃, carbon concentration is 0.004%, manganese concentration is 0.3
It became 0%, and although manganese was slightly oxidized, the carbon concentration reached 0.005% or less stably.

Comparative Example 1 Similarly, in a converter, 50 ton of molten steel 1 prepared by melting 0.04% of carbon and 0.33% of manganese from molten pig iron was tapped in a ladle 2 in an undeoxidized state. The composition of the molten steel in the ladle after tapping was 0.037% carbon and 410 ppm oxygen, and the temperature of the molten steel was 1670 ° C.

The refractory porous plug 3 placed at the bottom of the ladle was blown with Ar gas at a flow rate of 10 Nm ³ / hr, and the oxidizing slag 4 flowing out of the converter in the ladle was used as the inner wall of the ladle. In the pushed state, the ring-shaped refractory-made dipping pipe 7 was dipped as shown in FIG. 4 to secure a state in which there was no slag 4 in the dipping pipe. The inner diameter of the ladle 2 is 2.2 m (about 3.
Against 8m ^2), the inner diameter of the dip tube 7 is 2.0 m (about 3.
2 m ² ) about 80% of the oxidizing slag 4 on the surface of the molten steel was removed.

Next, as shown in FIG. 5, an upper blowing lance 8 is arranged at a height of 1.2 m from the surface of the molten steel, and Ar gas is blown onto the surface of the molten steel at a flow rate of 1000 Nm ³ / hr to perform decarburization treatment. Started. During this time, 10 Ar gas was introduced from the bottom of the ladle.
Blowing was continued at a flow rate of Nm ³ / hr and stirring was performed. When the state of the molten steel between the ladle 2 and the dip pipe 7 was examined 5 minutes on the way, it was just before solidification, so the decarburization treatment was unavoidably interrupted.

[0033]

As described above, according to the present invention, the tip of the lance tip is more expensive than the conventional expensive, large-scale, and extremely low carbon steel smelting method using a vacuum degassing apparatus that requires detailed maintenance. A conventional vacuum degassing device was used by reducing the slag on the surface of molten steel and controlling the oxygen concentration in the molten steel by the refractory disc installed in As in the case, 30
With a decarburization time of about a minute, it became possible to stably produce an ultra-low carbon steel having a carbon concentration of 0.005% or less.

According to the present invention, compared to the conventional method using a vacuum degassing apparatus, the temperature drop of molten steel is small and the adhesion of metal is small, so that stable and continuous processing is possible. Yes, and because maintenance is very easy, you can enjoy a reduction in refining processing costs. Needless to say, molten steel having a carbon concentration of 0.005% or more and 0.04% or less can be manufactured without using a vacuum degassing device as in the conventional case.

As described above, according to the present invention, it is possible to obtain excellent effects such as that an ultra-low carbon steel can be melted easily, reliably and inexpensively on an industrial scale.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a method of implementing the present invention.

FIG. 2 is an explanatory diagram showing an example of a method of implementing the present invention.

FIG. 3 is an explanatory diagram showing an example of a method of implementing the present invention.

FIG. 4 is an explanatory diagram showing an example of a comparative example for the present invention.

FIG. 5 is an explanatory diagram showing an example of a comparative example for the present invention.

[Explanation of symbols]

 1 Molten steel 2 Ladle 3 Porous plug 4 Oxidizing converter slag 5 Top blowing lance with a refractory disk at the tip 6 Immersion lance 7 Immersion pipe 8 Top blowing lance

Claims (1)

[Claims] 1. A lance provided with a refractory disk at its tip is used in a state where an inert gas is blown from the bottom of the ladle or an auxiliary lance to push the oxidizing slag on the molten steel surface toward the wall of the ladle. While spraying an inert gas from above, the lance is lowered to the vicinity of the molten steel surface to form a flow of the inert gas between the refractory disc and the molten steel surface, and the oxygen concentration in the molten steel is adjusted to 250 to 700 ppm. A method for producing ultra-low carbon steel, which comprises decarburizing the molten steel while controlling the range.