JPH04293711A - Method for melting extremely low carbon steel in converter - Google Patents

Abstract

PURPOSE:To provide a decarburizing method, in which the refining of steel is facilitated without much difference in comparison with decarburizing treatment time by means of the ordinary vacuum degassing device in the case of executing the decarburization treatment of the molten steel. CONSTITUTION:In the case of decarburizing the molten steel by dipping a refractory-made immersion body 7 into the zone removing oxidized slag on the surface of the molten steel 5 in a converter to form a specific space and blowing inert gas on the surface of molten steel having 0.04-0.03% carbon concn. and 400-700ppm oxygen concn. in the specific space, flowing velocity of the inert gas continuously blowing on the surface of molten steel in the specific space is >=20m/sec and also oxygen concn. in the molten steel is measured at any time, and by supplying the oxidizing gas or solid-state oxidizing source while blowing it into inner part so as to maintain the oxygen in the range of 250-700ppm concn., the decarburization can stably be exerted to the extremely low carbon range. The maintenance is facilitated and the extremely low carbon steel can surely be melted 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 in a converter without using a vacuum degassing device.

0002

BACKGROUND OF THE INVENTION Ultra-low carbon steels are known in which the carbon concentration in the steel sheet is reduced as much as possible in order to improve workability in thin steel sheets that are subjected to press working, such as steel sheets for automobiles. Conventionally, in order to melt this ultra-low carbon steel, molten steel is decarburized to a carbon concentration of about 0.04% in a converter, etc., and then received in a container such as a ladle in an undeoxidized state. In the RH method, DH method, etc., a part of the molten steel is exposed to a reduced pressure (vacuum) atmosphere through a process that uses an exhaust device such as a vacuum degassing device, and by lowering the pressure on the gas side, CO at the interface between the gas and the molten steel is removed. Under conditions that reduce the gas partial pressure, the carbon and oxygen in the molten steel are reacted as shown in the following formula, and after further decarburization, the molten steel is adjusted by adding an alloy to achieve the target molten steel composition. The manufacturing method is widely used.

[0003] C + O → CO
(1) Generally, when decarburizing molten steel, as shown in equation (1),
A method is used in which carbon in molten steel is reacted with oxygen to generate CO gas, and this gas is removed to the gas side. Oxygen is required in the molten steel to advance this decarburization reaction, so generally oxygen is introduced from above or below.
Alternatively, a pure oxygen converter system is adopted in which oxygen is supplied simultaneously 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 is stagnant, and instead, oxidation of iron, manganese, etc. occurs, resulting in a decrease in the yield of the molten steel and the effective removal of the molten steel. There are problems such as a decrease in the concentration of manganese, which is a component. Therefore, by lowering the partial pressure of carbon monoxide on the gas side so that the carbon in the molten steel is preferentially oxidized without oxidizing iron etc. even below the above carbon concentration, the reaction in equation (1) can be controlled to the right. Vacuum degassing equipment is widely used as a method for progressing in this direction.

In other words, in equation (2) shown below, if the partial pressure Pco of carbon monoxide on the gas side is made smaller, the carbon concentration can be made smaller even if the oxygen concentration in the molten steel is the same. be.

[0006]

[Math 1]

According to this method, the decarburization reaction progresses in the right direction as shown in equation (1) as the degree of reduced pressure (increases the degree of vacuum) is increased. By maintaining a high degree of vacuum, it is possible to produce ultra-low carbon steel with a carbon concentration of 0.005% or less. In these vacuum degassing devices, in order to further shorten the decarburization rate, oxidizing gas such as oxygen or carbon dioxide,
Alternatively, a method of adding a solid oxidation source such as iron oxide (for example, JP-A-49-34414, JP-A-51-1)
51211, JP 51-151212)
A method has also been developed in which a large amount of gas is blown into molten steel in order to increase the interfacial area of the reaction (Japanese Patent Laid-Open No. 52-5641).

[0008]

[Problems to be Solved by the Invention] The ultra-low carbon steel melting method using the vacuum degassing device described above is a very effective method from an equilibrium point of view for reducing carbon concentration, and is currently widely used. ing. However, as mentioned above, exposing a portion of the molten steel to a highly reduced pressure atmosphere requires a very large-scale and expensive vacuum equipment, and since the molten steel is processed under reduced pressure, refractories etc. are melted and To increase the adhesion of each fitting part in order to maintain a reduced pressure state at the bottom,
Requires careful maintenance.

[0009] Furthermore, when gas is blown into molten steel exposed to a reduced pressure atmosphere, when the gas leaves the molten steel surface, the molten steel scatters and deposits in the surrounding closed container.
Not only does this reduce the yield of molten steel, but it also requires a great deal of effort to remove the deposited metal. In view of the above problems,
The present invention was developed to solve these problems and to present a method for producing ultra-low carbon steel using existing equipment, which is equivalent to that produced using conventional vacuum degassing equipment. It is.

0010

[Means for Solving the Problems] The gist of the present invention is as follows. (1) The carbon concentration in the molten steel at the final stage of blowing in the top-bottom blowing converter is 0.04 to 0.03%, and the oxygen concentration is 400 to 700 pp.
At point m, the oxygen supply from the upper lance is interrupted, and after removing the slag on the surface of the molten steel by blowing inert gas from the bottom blowing tuyere placed at the bottom of the furnace, a refractory immersion body is immersed in this area. to form a specific space, and while constantly circulating the molten steel to the free surface from which the slag has been removed by blowing inert gas from the bottom blowing tuyere, inert gas is continuously sprayed onto the surface of the molten steel in the specific space. When decarburizing molten steel, the flow rate of the inert gas that is continuously blown onto the molten steel surface in the specified space is set to 20 m/sec or more, and the oxygen concentration in the molten steel is measured from time to time, and the concentration is 250 to 700 pp.
In a converter, an extremely low Carbon steel melting method.

(2) The carbon concentration in the molten steel at the final stage of blowing in the top-bottom blowing converter is 0.04 to 0.03%, and the oxygen concentration is 400%.
At ~700 ppm, the oxygen supply from the upper lance was interrupted, and after removing the slag on the surface of the molten steel by blowing inert gas from the bottom blowing tuyere placed at the bottom of the furnace, a refractory immersion body was placed in this area. A specific space is formed by immersion, and while the molten steel is constantly circulated to the free surface from which the slag has been removed by blowing inert gas from the bottom blowing tuyere, an inert gas is continuously supplied to the surface of the molten steel in the specific space. When decarburizing molten steel by spraying, the flow rate of the inert gas that is continuously sprayed onto the molten steel surface in the specific space is set to 20 m/sec or more, and the oxygen concentration in the molten steel is measured from time to time, and the concentration is 250 m/sec or more.
A solid oxidation source such as iron oxide is supplied while being blown into the molten steel so as to maintain the concentration within the range of ~700 ppm and to prevent oxides from accumulating on the molten steel surface in the specific space. A method for melting ultra-low carbon steel in a furnace.

0012

[Operation] The inventors of the present invention have stably reduced the carbon concentration in molten steel to 0.00% without using a conventional large-scale vacuum degassing device.
As a result of conducting research and development on a method to decarburize the steel to 0.005% or less, in order to reduce the partial pressure of carbon monoxide gas at the interface between the molten steel and the gas, the surface of the molten steel was decarburized using expensive and large-scale vacuum degassing equipment. Even without maintaining a reduced pressure state, if an inert gas such as argon or nitrogen is sprayed onto the molten steel interface to remove carbon monoxide gas at the interface and lower its partial pressure, the carbon concentration can be reduced to 0.0 at atmospheric pressure. It was discovered that the decarburization reaction progresses sufficiently until the carbon content reaches 0.005% or less, making it possible to produce ultra-low carbon steel.

[0013] Here, even in the case of the decarburization reaction using the converter method in which pure oxygen is supplied, a large amount of pure oxygen gas is supplied, and the partial pressure of carbon monoxide gas on the gas side is considered to be low. However, as mentioned earlier, when the carbon concentration falls below about 0.04%, oxidation of iron and manganese occurs preferentially over carbon, decarburization stalls, and the yield of molten steel decreases or This leads to a decrease in the concentration of manganese, an effective ingredient. The present inventors studied this phenomenon in the converter method and found that silicon, phosphorus, manganese, etc. are oxidized at the same time as carbon oxidation, and in order to stably fix these oxides, lime is mainly used as a component. It was revealed that the fact that slag for smelting was used for processing and the supply of pure oxygen gas were the causes of stagnation in decarburization.

[0014] In other words, when attempting to advance the decarburization reaction to a low carbon concentration region of 0.04% or less, the presence of oxidizing slag on the surface of the molten steel means that the reaction interface is effective for decarburization. This means that it is decreasing. Furthermore, it is necessary to control the oxygen supply rate commensurate with the carbon concentration, and if pure oxygen gas is supplied from above, the oxygen supply rate is too high, causing preferential oxidation of iron or manganese. These iron oxides or manganese oxides combine with the already existing slag and reduce the reaction interface where carbon and oxygen reactions occur.

Based on these research results, the present inventors first determined that the area occupied by the oxidizing slag on the surface of the molten steel in the ladle was reduced to 2.
Carbon concentration reduced to 0% or less 0.04-0.02%
If a mixed gas of oxidizing gas and inert gas adjusted to an oxygen partial pressure of 0.5 to 0.01 atm is sprayed onto the surface of the molten steel to decarburize the molten steel while suppressing oxidation of the molten steel, the carbon concentration can be reduced to 0. ．．
Discovered that it was possible to melt ultra-low carbon steel with a carbon content of 0.2% or less, and based on this knowledge, invented a method for melting ultra-low carbon steel (
(Patent Application No. 153454/1983).

[0016] Furthermore, the present inventors conducted research on whether it is possible to carry out this reaction within the converter in order to shorten the time required for decarburization in the ladle from which molten steel is tapped from the converter. We are progressing with development and have achieved a carbon concentration of 0.04 to 0.03% and an oxygen concentration of 40% in molten steel at the final stage of conventional normal blowing.
At the point of 0 to 700 ppm, the oxygen supply from the upper lance is interrupted, and after removing the slag on the surface of the molten steel by blowing inert gas from the bottom blowing tuyeres placed at the bottom of the furnace, a refractory immersion body is placed in this area. is immersed in the molten steel to form a specific space, and while the molten steel is constantly circulated to the free surface from which the slag has been removed by blowing inert gas from the bottom blowing tuyere, inert gas is continuously applied to the surface of the molten steel in the specific space. When decarburizing the molten steel by spraying it, the flow rate of the inert gas that is continuously sprayed onto the surface of the molten steel in the specific space is set to 20 m/sec or more, and the oxygen concentration in the molten steel is measured from time to time.
By controlling the oxygen concentration to maintain it within the range of 0 to 700 ppm and to prevent oxides from accumulating on the surface of the molten steel in the specific space, the carbon concentration can be stably reduced to 0.005% or less in a short period of time. It was confirmed that decarburization progressed to the extremely low carbon region.

Here, the carbon concentration in the molten steel at the final stage of blowing, which is the requirement of the present invention, is 0.04 to 0.03%, and the oxygen concentration is 40%.
The reason why the oxygen supply from the top blowing lance is interrupted at the point of 0 to 700 ppm is that when pure oxygen is supplied as mentioned above, when the carbon concentration is lower than 0.03%, the iron This is because a situation is reached in which the oxidation reaction of manganese proceeds preferentially.

[0018] The area for removing oxidizing slag on the surface of the molten steel in the converter is 20% of the surface area of the molten steel.
If the area is less than 20%, the supply of the inert gas to be blown to the reaction interface will be inhibited and it will be difficult to secure a reaction interface area effective for decarburization, so it is desirable to remove 20% or more. The easiest way to ensure 20% or more of the molten steel surface area free of this oxidizing slag is to remove the upper slag using only bottom blowing gas, but this method more reliably ensures a molten steel surface free of slag. Therefore, as mentioned above, stirring is performed using bottom-blown gas, and a ring-shaped refractory immersion body suspended above the furnace is lowered and immersed in the molten steel surface from which the upper slag has been removed. A method of exposing the molten steel surface free from molten steel, or attaching an iron plate cap (jinkasa-shaped cap) to the lower end of the ring-shaped immersed body made of refractory material, and stirring the immersed body with bottom-blown gas to remove the slag from the upper part. An effective method is to immerse the molten steel into the molten steel surface from which slag has been removed or reduced to expose the molten steel surface free of slag.

Next, the reason why an inert gas is used as the gas to be blown onto the surface of molten steel is that if the oxygen concentration in the blown gas exceeds 5%, oxidation of iron and manganese in molten steel will occur at the same time, causing oxidation in the molten state at the interface. Since these oxides reduce the effective interfacial area for decarburization, the rate of decarburization decreases and it takes longer to decarburize to a carbon concentration of 0.005% or less. This is because it has become clear that The inert gas used here is generally argon gas, but it is also possible to use helium gas, and if there is no problem with the steel material, nitrogen gas can also be used. They can also be used in combination.

On the other hand, in order to proceed with the decarburization reaction, oxygen is required to oxidize carbon, as shown in equation (1). Therefore, the oxygen concentration in molten steel before starting decarburization is:
The range is 400 to 700 ppm obtained when blowing is stopped at a carbon concentration of 0.04 to 0.03% in normal converter blowing,
While oxidizing carbon using the oxygen in the molten steel, the concentration of oxygen, which decreases as decarburization progresses, is measured at any time using known measuring means such as an oxygen concentration battery. If the oxygen concentration decreases, increase the concentration to 250 to 700 ppm to prevent new oxides from accumulating on the surface of the molten steel from which oxidizing slag was first removed and reducing the decarburization reaction interfacial area. need to be maintained.

The following means can be used to adjust the oxygen concentration in molten steel so that oxides are not newly accumulated on the surface of molten steel from which oxidizing slag has been removed as described above. (1) Pure oxygen gas, air, carbon dioxide gas, water vapor, etc. alone or a mixture of these oxidizing gases,
Alternatively, a mixed gas of these oxidizing gases and carbon monoxide gas, or a mixed gas of these oxidizing gases and inert gas, is blown into the molten steel through a bottom blowing tuyere or a refractory immersion lance, etc. How to supply.

(2) Either a single substance or a mixture of solid oxides such as iron oxide or manganese oxide is supplied into the molten steel through a bottom blowing tuyere or the like, or ores containing these components are inactivated. A method in which gas, etc. is used to supply the inside of molten steel through a bottom blowing tuyere or a refractory immersion lance. When supplying an oxidizing gas or a mixed gas of an oxidizing gas and an inert gas to the inside of the molten steel, or when supplying a solid oxidation source such as iron oxide to the inside of the molten steel, the oxygen concentration must be kept almost constant. Oxygen may be supplied continuously little by little so that it can be controlled, or a certain amount of oxygen may be supplied all at once in a short period of time.

The reason why the flow velocity of the inert gas sprayed onto the surface of the molten steel from which oxidizing slag has been removed is ensured at 20 m/sec or more is that if it is less than 20 m/sec, the amount of carbon monoxide gas generated by decarburization will be reduced. This is because the pressure cannot be maintained at a low level for decarburization to proceed. By ensuring the flow velocity at the interface is 20 m/sec or more, a state similar to that under reduced pressure is ensured at the interface even under atmospheric pressure.
The greater the flow velocity, the greater the effect.

The reason why it is necessary to control the oxygen concentration in molten steel to 250 ppm or more is because the decarburization rate tends to decrease when the concentration is less than 700 ppm.
The reason for controlling the following is that once this concentration is exceeded, the effect of oxygen concentration on the decarburization rate is no longer observed, and the amount of minute inclusions that are generated when this oxygen is deoxidized later increases. Also due to unfavorable things.

[0025] By this method, the molten steel surface from which oxidizing slag has been removed for the purpose of securing the reaction interface area for decarburization,
Furthermore, by eliminating the generation and accumulation of oxides and removing carbon monoxide gas at the interface by gas blowing, the decarburization rate can always be maintained at a high level. For the above reasons, the carbon concentration is set to 0.
．． If you want the decarburization to proceed as low as 0.005% or less and in a short time, the surface area of the molten steel occupied by the oxidizing slag should be increased by blowing gas to increase the reaction interface area where the decarburization proceeds. Needless to say, it is effective to make the molten steel small and to stir the molten steel strongly so that carbon in the molten steel is constantly supplied to the reaction interface. In order to stir the molten steel here, it is preferable to increase the amount of gas supplied from the bottom blowing tuyeres of the converter, and the gas for this purpose is preferably an inert gas similar to the gas blown from the top. Pure oxygen gas, air, carbon dioxide gas, water vapor, etc. alone or a mixture of these gases, a mixture of these oxidizing gases and carbon monoxide gas, or a mixture of these oxidizing gases and carbon monoxide gas to control the oxygen concentration. A mixed gas of an inert gas may be used, or an inert gas for supplying a solid oxidation source such as iron oxide to the inside of the molten steel may also be used. When using a gas mixed with an oxidizing gas as the molten steel stirring gas, the molten steel is heated through a bottom blowing tuyere or a refractory immersion lance to adjust the oxygen concentration in the molten steel. It can replace part or all of the oxidizing gas that is blown into the interior.

[0026]

[Example] Example 1 In a top-bottom blowing converter 1 with a furnace capacity of 250 tons, 0.038% carbon, 0.34% manganese, and 470 ppm oxygen were extracted from hot metal.
As shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and argon gas 4 is injected at a rate of 300 Nm3 /
hr flow rate, and with this gas pushing the slag 6 on the surface of the molten steel 5 towards the inner wall of the converter, a ring-shaped refractory immersion tube 7 installed in the furnace in advance is immersed, and the inside of the immersion tube is immersed. The slag 6 was discharged out of the dipping tube. The inner diameter of the converter 1 was 5 m (area: about 20 m2), whereas the inner diameter of the immersion tube 7 was 2.5 m (area: 5 m2), and 25% of the oxidizing slag on the surface of the molten steel was removed. Next, argon gas 8 is supplied from the top blow lance 2 at a flow rate of 5000Nm3/hr.
was sprayed onto the surface of the molten steel to perform decarburization treatment for 12 minutes. When we measured the gas flow velocity on the surface of the molten steel at this time, it was approximately 70m/
It was sec. During this time, argon gas 4 was continuously blown in from the tuyere 3 at the bottom of the furnace at a flow rate of 300 Nm<3>/hr to perform stirring. When the oxygen concentration was measured 3 minutes after the decarburization process, it showed 250 ppm, so 50 N of pure oxygen gas was added to the argon gas 4 from the blowing tuyere 3 at the bottom of the furnace.
m3/hr mixed and blown in for 5 minutes to bring the oxygen concentration to 35
It was raised to 0 ppm. After this decarburization treatment, the carbon concentration of the molten steel was 0.003%, and the manganese concentration was 0.30%. Although the manganese was slightly oxidized, the carbon concentration remained stable at 0.
．． It reached 0.005% or less. Example 2 In a top-bottom blowing converter 1 with a furnace capacity of 250 tons, 0.036% carbon, 0.32% manganese, and 500 ppm oxygen were extracted from hot metal.
As shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and argon gas 4 is injected at a rate of 200 Nm3 /
hr flow rate, and with this gas pushing the slag 6 on the surface of the molten steel 5 towards the inner wall of the converter, a ring-shaped refractory immersion tube 7 installed in the furnace in advance is immersed, and the inside of the immersion tube is immersed. The slag 6 was discharged out of the dipping tube. While the inner diameter of the converter 1 is 5 m (area approximately 20 m2), the immersion tube 7
The inner diameter of the molten steel was 2.5 m (area: 5 m2), and 25% of the oxidizing slag on the surface of the molten steel was removed. Next, a mixed gas 8 of argon gas and nitrogen gas is supplied from the top blow lance 2 at a flow rate of 60.
It was sprayed onto the surface of the molten steel at a rate of 00 Nm3/hr to perform decarburization treatment for 10 minutes. The gas flow velocity on the surface of the molten steel at this time was measured and was approximately 70 m/sec. During this time, argon gas 4 was continuously blown into the furnace from the tuyere 3 at the bottom of the furnace at a flow rate of 200 Nm3/hr to perform stirring. 3 from the above decarburization process
When the oxygen concentration was measured after a minute had passed, it showed 280 ppm, so the gas 4 from the blowing tuyere 3 at the bottom of the furnace was changed to a mixed gas of pure oxygen and carbon dioxide, and the oxygen concentration was 200 Nm3.
Blow for 5 minutes at a flow rate of /hr to bring the oxygen concentration to 350pp.
It was controlled to m. The carbon concentration of the molten steel after this decarburization treatment is 0.
004%, the manganese concentration was 0.30%, and although the manganese was slightly oxidized, the carbon concentration remained stable at 0.005%.
The following has been reached. Example 3 In a top-bottom blowing converter 1 with a furnace capacity of 250 tons, 0.040% carbon, 0.36% manganese, and 400 ppm oxygen were extracted from hot metal.
As shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and argon gas 4 is injected at a rate of 200 Nm3 /
hr flow rate, and with this gas pushing the slag 6 on the surface of the molten steel 5 towards the inner wall of the converter, a ring-shaped refractory immersion tube 7 installed in the furnace in advance is immersed, and the inside of the immersion tube is immersed. The slag 6 was discharged out of the dipping tube. The inner diameter of the converter 1 was 5 m (area: about 20 m2), whereas the inner diameter of the immersion tube 7 was 2.5 m (area: 5 m2), and 25% of the oxidizing slag on the surface of the molten steel was removed. Next, argon gas 8 is supplied from the top blow lance 2 at a flow rate of 6000Nm3/hr.
was sprayed onto the surface of the molten steel to perform decarburization treatment for 12 minutes. When we measured the gas flow velocity on the surface of the molten steel at this time, it was approximately 75 m/
It was sec. During this time, argon gas 4 was continuously blown in from the tuyere 3 at the bottom of the furnace at a flow rate of 200 Nm3/hr to perform stirring. When the oxygen concentration was measured 3 minutes after the decarburization process, it was found to be 260 ppm. Therefore, as shown in FIG. The oxygen concentration was controlled at 300 ppm by blowing at a flow rate of /hr for 5 minutes. After this decarburization treatment, the carbon concentration of the molten steel was 0.003%, and the manganese concentration was 0.30%, and although manganese was slightly oxidized, the carbon concentration stably reached 0.005% or less. Example 4 In a top-bottom blowing converter 1 with a furnace capacity of 250 tons, 0.035% carbon, 0.31% manganese, and 520 ppm oxygen were extracted from hot metal.
As shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and argon gas 4 is injected at 250 Nm3 /
hr flow rate, and with this gas pushing the slag 6 on the surface of the molten steel 5 towards the inner wall of the converter, a ring-shaped refractory immersion tube 7 installed in the furnace in advance is immersed, and the inside of the immersion tube is immersed. The slag 6 was discharged out of the dipping tube. The inner diameter of the converter 1 was 5 m (area: about 20 m2), whereas the inner diameter of the immersion tube 7 was 2.5 m (area: 5 m2), and 25% of the oxidizing slag on the surface of the molten steel was removed. Next, argon gas 8 is supplied from the top blow lance 2 at a flow rate of 6000Nm3/hr.
was sprayed onto the surface of the molten steel to perform decarburization treatment for 13 minutes. When we measured the gas flow velocity on the surface of the molten steel at this time, it was approximately 70m/
It was sec. During this time, argon gas 4 was continuously blown in from the tuyere 3 at the bottom of the furnace at a flow rate of 200 Nm3/hr to perform stirring. When the oxygen concentration was measured 3 minutes after the decarburization process, it was found to be 290 ppm. Therefore, as shown in Figure 3, an auxiliary refractory immersion lance 9 was immersed, and argon gas 100 Nm3/hr was used as a carrier gas for oxidation. Iron 10 at 20 kg/min for 5 minutes (iron oxide total 10
0 kg) and controlled the oxygen concentration to 420 ppm. After this decarburization treatment, the carbon concentration of the molten steel was 0.004%, and the manganese concentration was 0.30%, and although manganese was slightly oxidized, the carbon concentration stably reached 0.005% or less. Comparative Example 1 In the same top-bottom blowing converter 1 with a furnace capacity of 250 tons, hot metal contains 0.034% carbon, 0.29% manganese, and 580% oxygen.
When the blowing reaches ppm, as shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and 200N of argon gas 4 is introduced from the four gas blowing tuyeres 3 installed at the bottom of the furnace.
The gas was blown at a flow rate of m3/hr, and the slag 6 on the surface of the molten steel 5 was pushed toward the inner wall of the converter.
A ring-shaped refractory immersion tube 7 previously installed in the furnace was immersed, and the slag 6 inside the immersion tube was discharged to the outside of the immersion tube. The inner diameter of the converter 1 is 5 m (area approximately 20 m2), while the inner diameter of the immersion tube 7 is 2.5 m (area 5 m2).
25% of the oxidizing slag on the surface of the molten steel was removed. Next, argon gas 8 is supplied from the top blow lance 2 at a flow rate of 8000 Nm3.
/hr to perform decarburization treatment on the surface of the molten steel for 10 minutes. The gas flow velocity on the surface of the molten steel at this time was measured and was approximately 80 m/sec. During this time, argon gas 4 was continuously blown in from the tuyere 3 at the bottom of the furnace at a flow rate of 200 Nm3/hr to perform stirring. Three minutes after the decarburization process, the oxygen concentration was measured and showed 300 ppm, so only argon gas was continued to be blown from the top blowing lance 2. After the decarburization was completed, the oxygen concentration of the molten steel was measured again and it was 200.
ppm, and the carbon concentration was 0.007% and the manganese concentration was 0.27%, and the carbon concentration did not reach 0.005% or less within this time. Comparative Example 2 In a top-bottom blowing converter 1 with a furnace capacity of 250 tons, 0.039% carbon, 0.31% manganese, and 520 ppm oxygen were extracted from hot metal.
As shown in Fig. 1, the oxygen supply from the top blowing lance 2 is stopped, and argon gas 4 is injected at a rate of 200 Nm3 /
hr flow rate, and with this gas pushing the slag 6 on the surface of the molten steel 5 towards the inner wall of the converter, a ring-shaped refractory immersion tube 7 installed in the furnace in advance is immersed, and the inside of the immersion tube is immersed. The slag 6 was discharged out of the dipping tube. The inner diameter of the converter 1 was 5 m (area: about 20 m2), whereas the inner diameter of the immersion tube 7 was 2.5 m (area: 5 m2), and 25% of the oxidizing slag on the surface of the molten steel was removed. Next, argon gas 8 is supplied from the top blow lance 2 at a flow rate of 5000Nm3/hr.
was sprayed onto the surface of the molten steel to perform decarburization treatment for 10 minutes. When we measured the gas flow velocity on the surface of the molten steel at this time, it was approximately 60m/
It was sec. During this time, argon gas 4 was continuously blown in from the tuyere 3 at the bottom of the furnace at a flow rate of 200 Nm<3>/hr to perform stirring. When the oxygen concentration was measured 3 minutes after the decarburization process, it showed 300 ppm, so 500 Nm3 of pure oxygen gas was added to the argon gas from the top blowing lance 2.
/hr mixing (oxygen gas concentration 10%) and continued blowing for the remaining 5 minutes. Finally, the oxygen concentration in the molten steel rose to 750 ppm. The carbon concentration of molten steel after this decarburization treatment is 0.0
10%, the manganese concentration was 0.21%, the manganese was oxidized, and the carbon concentration did not reach 0.005% or less.

[0027]

[Effects of the Invention] As described above, according to the present invention, compared to the conventional method for producing ultra-low carbon steel using a vacuum degassing device that is expensive, large-scale, and requires careful maintenance, this method is more effective. It is possible to simply reduce the slag on the surface of the molten steel at the final stage of normal converter blowing, without having to modify or install new degassing equipment, etc. By controlling the oxygen concentration of
It has become possible to produce ultra-low carbon steel with a carbon concentration of 0.005% or less in a decarburization time of about 30 minutes. Furthermore, according to the present invention, compared to the conventional method using a vacuum degassing device, ultra-low carbon steel can be produced using only one reaction vessel called a converter, the temperature drop of the molten steel is small, and There is almost no adverse effect on the next blowing due to gold removal, etc., maintenance is extremely easy, and processing costs can be reduced. In addition, decarburization to a carbon concentration of 0.005% or more and 0.03% or less is also possible.
It goes without saying that there is no oxidation of manganese, etc., and that it can be melted without using a conventional vacuum degassing device.

As described above, according to the present invention, excellent effects such as the ability to easily, reliably, and inexpensively produce ultra-low carbon steel on an industrial scale can be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing embodiments of the present invention and a comparative example.

FIG. 2 is an explanatory diagram showing another embodiment of the present invention and a comparative example.

FIG. 3 is an explanatory diagram showing still another embodiment of the present invention and a comparative example.

[Explanation of symbols]

1 Top-bottom blowing converter 2 Top blow lance 3 Bottom-blown tuyere 4 Blowing gas 5 Molten steel 6 Converter slag 7 Immersed refractory body 8. Blowing gas 9 Immersion lance made of refractory material 10 Solid oxidation source

Claims (2)

[Claims] Claim 1: The carbon concentration in the molten steel at the final stage of blowing in a top-bottom blowing converter is 0.04 to 0.03%, and the oxygen concentration is 400 to 70%.
At 0 ppm, the oxygen supply from the upper lance is interrupted,
After removing the slag on the surface of the molten steel by blowing inert gas from the bottom blowing tuyere located at the bottom of the furnace, a refractory immersion body is immersed in this area to form a specific space, and When decarburizing the molten steel by continuously blowing inert gas onto the surface of the molten steel in the specific space while constantly circulating the molten steel to the free surface from which the slag has been removed by blowing inert gas, the molten steel in the specific space is decarburized. The flow rate of the inert gas that is continuously blown onto the surface is set to 20 m/sec or more, and the oxygen concentration in the molten steel is measured from time to time, and the concentration is 250 to 70 m/sec.
The extremely low temperature in a converter is characterized by supplying an oxidizing gas while blowing it into the molten steel so as to maintain it within a range of 0 ppm and to prevent oxides from accumulating on the surface of the molten steel in the specific space. Carbon steel melting method. 2. The carbon concentration in the molten steel at the final stage of blowing in the top-bottom blowing converter is 0.04 to 0.03%, and the oxygen concentration is 400 to 70%.
At 0 ppm, the oxygen supply from the upper lance is interrupted,
After removing the slag on the surface of the molten steel by blowing inert gas from the bottom blowing tuyere located at the bottom of the furnace, a refractory immersion body is immersed in this area to form a specific space, and When decarburizing the molten steel by continuously blowing inert gas onto the surface of the molten steel in the specific space while constantly circulating the molten steel to the free surface from which the slag has been removed by blowing inert gas, the molten steel in the specific space is decarburized. The flow rate of the inert gas that is continuously blown onto the surface is set to 20 m/sec or more, and the oxygen concentration in the molten steel is measured from time to time, and the concentration is 250 to 70 m/sec.
A converter characterized in that a solid oxidation source such as iron oxide is supplied while being blown into the inside of the molten steel so as to maintain the concentration within the range of 0 ppm and to prevent oxides from accumulating on the surface of the molten steel in the specific space. A method for producing ultra-low carbon steel.