JP2991519B2 - Manufacturing method of ultra-low carbon steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RH recirculation type degassing apparatus (hereinafter referred to as RH), wherein carbon in molten steel (hereinafter referred to as RH) is used.
[C]), for example, 0.001
The present invention relates to an efficient and economical method for removing ultra-low carbon steel by removing up to wt%.

[0002]

2. Description of the Related Art In RH, a method of blowing an inert gas into molten steel in a vacuum chamber is often used to increase a degassing rate. The method disclosed in JP-A-57-110611 is one example. In this method, a gas outlet is provided at the bottom of the vacuum tank, and an inert gas is blown into the molten steel from the gas outlet to promote decarburization. In addition, a method of blowing an inert gas from a gas outlet provided on a side wall of the vacuum chamber may be used.

[0003]

In the method of blowing an inert gas into molten steel from a gas outlet installed on the bottom or side wall of a vacuum chamber, in order to prevent molten steel from entering the gas outlet, While the gas jet is immersed in the molten steel,
The gas must always flow. Therefore, the cost of degassing increases due to an increase in the amount of inert gas used and an increase in the amount of water vapor used to increase the amount of evacuation. Furthermore, the injection of a large amount of gas in the high carbon concentration region promotes the generation of splash, and it is difficult to melt ultra-low carbon steel due to the [C] pickup by remelting the metal adhered to the inner wall of the vacuum chamber.

On the other hand, according to a method in which a porous plug is provided at the bottom of the vacuum chamber and an inert gas is blown, the inert gas can be blown only when necessary, so that the above-mentioned problems can be prevented. . However, when injecting gas using a porous plug, it is difficult to inject sufficient gas to increase the decarburization rate because the upper limit of the gas injection flow rate is limited.

Injecting gas from the bottom of the vacuum tank impedes the flow of molten steel in the vacuum tank from the rising pipe side to the downcoming pipe side, and reduces the amount of molten steel reflux, which is disadvantageous for decarburization. Become.

[0006]

In order to solve the above-mentioned problems, the height of the lance can be changed to the vacuum tank canopy at RH.
And a lance with a horizontal gas discharge direction is installed, and the concentration of [C] is 0.005 wt% from the start of the decarburization treatment.
In the [C] concentration range up to, the lance is not immersed in molten steel,
And decarburizing without flowing gas from the lance,
In the [C] concentration range where the [C] concentration is 0.005 wt% or less, as shown in FIG. 2, the lance is immersed in molten steel between the riser tube and the inner wall of the vacuum tank when viewed from above the vacuum tank. To that end, they invented a method of blowing an inert gas into molten steel.

[0007]

Hereinafter, the present invention will be described in detail. When the [C] concentration is higher than 0.005 wt% in RH,
Even if an inert gas is blown into the vacuum chamber, the increase in the decarburization rate is small. That is, in this [C] concentration region, the amount of CO gas generated from inside the molten steel due to the decarburization reaction is much larger than the flow rate of the blown gas. The contribution of the increase of the area and the stirring of the molten steel to the promotion of decarburization is very small. Therefore,
In this [C] concentration region, the inert gas is blown to increase the pressure in the vacuum chamber, so that the decarburization speed is reduced, and the amount of inert gas used and the amount of water vapor used for evacuation are reduced. And the cost of decarburization increases. Furthermore, an increase in the amount of [C] pick-up due to an increase in the amount of generated splash reduces the apparent decarburization rate and causes the attained [C] concentration to increase.

On the other hand, in the [C] concentration range where the [C] concentration is 0.005 wt% or less, the amount of generated CO bubbles is reduced, and the area of the gas-liquid reaction interface and the stirring power of molten steel are reduced, so that decarburization occurs. Speed decreases. In this region, blowing the inert gas into the molten steel in the vacuum chamber is effective to increase the gas-liquid reaction interface area and increase the molten steel stirring power, thereby increasing the decarburization rate. Here, the direction in which the inert gas is blown is also important, and the direction in which the flow of the molten steel in the vacuum chamber is not hindered, that is, blowing from the riser pipe toward the downpipe is effective for promoting decarburization. is there.

Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. FIG. 1 illustrates an RH embodying the present invention.
FIG. The ascending pipe 2A and the descending pipe 2B at the lower end of the vacuum chamber 1 are immersed in the molten steel 3, evacuated from the exhaust port 4, and blown with an inert gas from the gas inlet 5 provided in the middle of the immersion pipe 2A. A lance 6 for blowing an inert gas is provided on the canopy of the vacuum tank 1 in a device for sucking and returning the molten steel 3 into the vacuum tank 1. In performing decarburization treatment of molten steel using the present apparatus, as shown in FIG. 1A, a lance 6 is connected to molten steel 7 in a vacuum chamber from the start of decarburization treatment until the [C] concentration reaches 0.005 wt%. Perform decarburization treatment without immersion and without flowing inert gas, [C]
In the [C] concentration range where the concentration is 0.005 wt% or less, the lance 6 is viewed from above the vacuum chamber as shown in FIGS. And an inert gas is blown toward the hatched portion in FIG. 2 to perform a decarburization treatment. Here, FIG. 2 is a view of a cross section of the vacuum chamber viewed from above, and a hatched portion in FIG. 2 indicates a tangent line drawn from the lance 6 to the downcomer 2 </ b> B and an extension thereof and the inner wall of the vacuum chamber 1. It is the part enclosed.

In the method of the present invention, the flow rate of the inert gas blown into the molten steel 7 from the lance 6 is shown in FIG. 3 in comparison with the conventional method. According to the method of the present invention, the amount of inert gas blown into the vacuum chamber can be significantly reduced. Since the flow rate of the inert gas blown into the vacuum chamber can be reduced, the amount of evacuation can be reduced, and the amount of water vapor used for evacuation can be reduced.

In addition, since the inert gas is not blown in the [C] concentration region where the [C] concentration exceeds 0.005 wt%, the pressure in the vacuum chamber is not increased, and the inert gas is kept in the [C] concentration range. The decarburization speed is higher than in the gas blowing method.

Further, according to the method of the present invention, an inert gas is not blown into the molten steel in the vacuum chamber at the initial stage of the decarburization treatment, so that the amount of splash generated is small and the amount of metal adhering to the inner wall of the vacuum chamber is small. Therefore, [C] from bullion at the end of decarburization
Pickup is also small. Furthermore, since inert gas is blown in a direction that does not hinder the flow of the molten steel in the vacuum chamber and the molten steel is stirred, decarburization can be promoted. As described above, as shown in FIG. 4, the [C] concentration is 0.0025 wt.
%, The decarburization rate apparently increases, and the ultimate [C] concentration also decreases.

As described above, in order to reduce the amount of inert gas used and the amount of steam used and to produce ultra-low carbon steel, the inert gas is always evacuated during the decarburization process as in the conventional method. It is not possible to cope with the method of blowing into molten steel in the tank, and it is more advantageous to cope with the method of blowing an inert gas into the molten steel in the vacuum tank as necessary, as in the method of the present invention.

[0014]

EXAMPLES The initial component was [C]; 0.04 wt%, [S
i]; 0.1 wt% or less, [Mn]: 0.01 to 0.5
wt%, [P]; 0.005 to 0.02 wt%,
[S]; 0.003 to 0.015 wt%, [Al];
RH of 300 tons of molten steel at 0.002 wt% or less
The decarburization treatment was performed using.

As shown in FIG. 3, the lance 6 installed on the canopy of the vacuum tank 1 is immersed in the molten steel 7 in the vacuum tank for about 8 minutes from the start of the decarburization treatment until the [C] concentration reaches 0.005 wt%. The decarburization treatment was performed without flowing inert gas, and the [C] concentration was 0.005 wt% or less. As shown in FIG. It was immersed in molten steel 7 between the inner wall and Ar gas of 2000 Nl / min was blown into the molten steel toward the hatched portion in FIG.
Table 1 shows the amount of steam used for 20 minutes from the start of the decarburization treatment, and FIG. 4 shows the change over time of the [C] concentration.

Comparative Example 1 is a case where Ar gas was blown into molten steel at the flow rate of 2000 Nl / min from the start of the decarburization treatment from the gas outlet provided at the bottom of the vacuum chamber. The reflux Ar gas flow rate was 2000 Nl / min.

In Comparative Example 2, the lance 6 installed on the canopy of the vacuum tank 1 was not immersed in the molten steel 7 in the vacuum tank for about 8 minutes from the start of the decarburization treatment until the [C] concentration was 0.005 wt%. And decarburizing without flowing inert gas, [C]
When the concentration is 0.005 wt% or less, the lance 6 is immersed in molten steel 7 between the riser tube and the inner wall of the vacuum tank when viewed from above the vacuum tank,
This is a case in which 2000 Nl / min of Ar gas is blown into molten steel toward portions other than the hatched portions in FIG. As shown in Table 1, according to the method of the present invention, the amount of Ar gas used and the amount of water vapor used were reduced as compared with Comparative Example 1, and as compared with Comparative Examples 1 and 2, as shown in FIG. Thus, molten steel having a lower [C] concentration could be produced.

[0018]

[Table 1]

[0019]

According to the method of the present invention, the amount of inert gas and steam used in the decarburization treatment in RH is reduced, and
[C] The ultra-low carbon steel having a concentration of 0.001 wt% or less can be easily melted.

[Brief description of the drawings]

FIGS. 1A and 1B are views showing an example of an RH type reflux degassing apparatus embodying the present invention.

FIG. 2 is a diagram showing a cross section of the vacuum tank viewed from above the vacuum tank.

FIG. 3 is a diagram showing a change with time of the flow rate of an inert gas injected into molten steel in a vacuum chamber according to the present invention and a comparative example.

FIG. 4 is a graph showing the change over time of the [C] concentration in Examples and Comparative Examples of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Vacuum tank 2A ... Up pipe 2B ... Down pipe 3 ... Molten steel 4 ... Exhaust port 5 ... Gas inlet for recirculation 6 ... Lance 7 ... Molten steel in a vacuum tank 8 ... Molten steel ladle

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Muneyasu Nasu 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Shigeaki Ogibayashi 1 Kimitsu City, Kimitsu City Nippon Steel Corporation Kimitsu Inside the steelworks (56) References JP-A-1-246314 (JP, A) JP-A-2-217412 (JP, A) JP-A-53-92319 (JP, A) JP-A-4-289113 (JP, A A) JP-A-4-289114 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21C 7/10 C21C 7/068

Claims (1)

(57) [Claims] 1. A decarburization process in a RH reflux type degassing device, in which a lance with a variable lance height and a horizontal gas discharge direction is installed in a vacuum tank canopy when performing a decarburization process. 0.005w [C] concentration from the start of
In the [C] concentration range up to t%, the lance is not immersed in the molten steel, and decarburization is performed without flowing gas from the lance, and the [C] concentration is 0.005 wt% or less. In the range, the lance is immersed in the molten steel between the riser tube and the inner wall of the vacuum tank as viewed from above the vacuum tank as shown in FIG. 2, and an inert gas is blown into the molten steel toward the hatched portion. Of ultra-low carbon steel.