JPH05156343A - Production of extremely low nitrogen steel - Google Patents

Abstract

PURPOSE:To provide and efficient and simple denitrogenizing method for molten steel at a low cost which removes nitrogen [N] contained in the molten steel to extremely low level so as to produce the extremely low nitrogen steel. CONSTITUTION:At the time of executing the denitrogenizing treatment of the molten steel while decarbonizing the molten steel in reduced pressure, the decarbonizing speed in this molten steel or the decarbonizing rate in the molten steel converted into the unit time is secured to >=0.005wt.%/min, and while controlling [O] <=0.030wt.%, carbon concn. [C] in the molten steel is always held to >=0.010wt.%, and by directly blowing oxidizing gas into the molten steel, the extremely low nitrogen steel can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to remove nitrogen [N] contained in molten steel to a very small amount, and to produce an extremely low nitrogen steel. It relates to the method of sequestration.

[0002]

2. Description of the Related Art Nitrogen contained in steel is a thin steel sheet for automobiles,
In the case of a steel sheet used as a thin steel sheet for beverage cans, it should be in a very small amount in order to improve workability and prevent aging, and in the case of steel for ultrafine wire steel cords, to improve ductility. Is required. Generally, in the steel industry, denitrification treatment of molten steel is performed, for example, in the 3rd edition Iron and Steel Handbook II
It carries out using various vacuum refining equipment as shown in pages 71-685. In this case, oxygen [O] contained in the molten steel or an oxidation source such as iron ore Fe _x O _y and oxygen gas O ₂ is used to remove carbon [C] contained in the molten steel by the following reaction. The gas-liquid interface due to the removed CO gas bubbles is used as a denitrification reaction site.

[C] + [O] = CO (gas) …………………………………… (1) y [C] + Fe _x O _y = yCO (gas) + x [Fe] …… (2) That is, nitrogen [N] of molten steel is removed by the following reaction at the gas-liquid interface. [N] = 1 / 2N ₂ (gas) ……………………………… (3) However, iron and steel 73rd year No. 11 No. 1559-15
As described on page 66, the rate of the denitrification reaction represented by the formula (3) becomes extremely small as the [O] and [S] concentrations increase. However, as is well known in the art, increasing the decarburization rate of molten steel increases the denitrification rate of molten steel (Iron and Steel No. 63, No. 13, 2077-2).
086). Therefore, based on these ideas, researches utilizing the decarburization reaction of molten steel by adding iron ore have been recently reported. For example, Iron and Steel No. 73, No. 2, 313-320.
The page describes that denitrification of molten steel is performed by spraying a certain amount of iron ore powder having a size of 0.1 mm or less onto molten steel having a high carbon concentration under reduced pressure to decarburize the molten steel. Further, one of the inventors of the present invention is iron and steel 74th No. 3rd 441-
On page 448, it is clarified that when the iron ore supply rate is changed as a means for changing the decarburization rate, the denitrification rate increases almost in proportion to the supply rate.

However, in the above method, in order to accelerate the decarburization reaction, solid oxide such as iron ore and manganese ore is added to the molten steel to secure the [O] concentration, so that the temperature of the molten steel decreases. It was necessary to separately heat the molten steel. further,
When the [C] concentration decreases and the decarburization reaction does not proceed actively, the [O] concentration increases and the denitrification rate becomes extremely small. To remove [N] to an extremely low concentration, molten steel The treatment time for denitrification must be extended, and the temperature of the molten steel also decreases in such cases, so the temperature of the molten steel to be denitrified in advance in the converter or electric furnace in the previous step It has to be dealt with by tapping at a high temperature or reheating the molten steel in the next step. That is, if the tapping temperature before the denitrification treatment is set to a high temperature, the refractory in the converter or the electric furnace is melted and damaged, the refractory basic unit becomes large, and the cost for the denitrification treatment becomes high. This is inefficient and uneconomical, and it is extremely difficult to stably produce ultra-low nitrogen steel.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an economical method for denitrifying molten steel for the production of extremely low nitrogen molten steel.

[0006]

Means and Actions for Solving the Problem The gist of the present invention is that a decarburization rate or a unit for carrying out denitrification treatment of a molten steel while guiding a portion of the molten steel to a decompression tank while decarburizing the molten steel. The decarburization amount of molten steel converted per hour is 0.
005 [wt% / min] or more, and the oxygen concentration [O] of the molten steel is controlled to 0.030 wt% or less, while the carbon concentration [C] of the molten steel is kept at 0.010 wt% or more, the molten steel is oxidizable. This is a method for melting ultra-low nitrogen steel, which is characterized by blowing gas directly.

At this time, the atmospheric pressure of the vacuum chamber in contact with the molten steel free surface is set to 70 mmHg or less. At this time, in order to efficiently denitrify molten steel, minimize the amount of molten steel splash accompanying decarburization and reduce the amount of steel that adheres to the reaction vessel and solidifies (hereinafter referred to as metal). In particular, it is preferable to control the atmospheric pressure of the vacuum chamber within the range of 5 to 30 mmHg.

Further, the CO gas generated by the decarburization reaction is
By supplying oxygen gas into the vacuum chamber and burning it,
By reducing the decrease in molten steel temperature and ensuring the temperature of the refractory in the vacuum tank, the amount of the adhered metal can be reduced. That is, the root of the technical idea of the present invention is to supply the oxygen source to the molten steel and suppress the oxygen concentration [O] of the molten steel to the lowest possible concentration when denitrifying the molten steel according to the formula (3). [C]
To increase the decarburization rate, and to remove C by the decarburization reaction.
This is to increase the amount of O gas bubbles, increase the gas / liquid interface area per unit weight of molten steel, and increase the number of denitrification reaction sites. At this time, in order to continuously advance the decarburization reaction and to blow the oxidizing gas as the [O] source directly into the molten steel in order to minimize the decrease in the molten steel temperature, and to secure the [C] concentration. In some cases, a carbon source is supplied to the molten steel to increase the gas / liquid interface area where the molten steel and the CO gas bubbles are in contact with each other, and at the same time suppress the increase of the [O] concentration.

At this time, a part or all of the [C] consumed by decarburization is supplemented with graphite, coke, coal or a hydrocarbon gas. These carbon sources supplied to molten steel [C]
As CO gas, reacts with the supplied oxidizing gas to generate CO gas bubbles, continuously suppresses the increase in [O] concentration, and promotes denitrification while removing the adverse effect of [O] on the denitrification reaction. Let

When the decarburization reaction is continuously carried out by continuously supplying the oxidizing gas and the carbon source to the molten steel, the addition rate Fo (mol / min) of oxygen O in the oxidizing gas and the carbon source The carbon C addition rate Fc (mol / min) is preferably added in the following range. 0.5 ≦ (Fc / Fo) ≦ 1.5 …………………………………… (4) When 0.5> (Fc / Fo), the oxygen source becomes excessive, and eventually The carbon source is insufficient, the [O] concentration rises, and the denitrification reaction is hindered. When (Fc / Fo)> 1.5, the [C] concentration of the molten steel increases, which is uneconomical.

Since the decarburization reaction within the scope of the present invention is the rate-determining supply of the oxygen source, the decarburization rate Vc is uniquely determined by the rate of supply of the oxidizing gas. Therefore, the value of Vc is 0.00
The addition rate of the oxygen source added to secure 5 [wt% / min] or more can be stoichiometrically determined according to the decarburization reaction formula as shown in formula (2). Generally, the oxidizing gas is preferably oxygen gas, but carbon dioxide gas may be used alone or a mixture of these gases may be used. Further, even if these gases contain water vapor, the effect is the same.

In carrying out the present invention, the carbon source added to the molten steel may be a solid carbon source such as graphite, coke or coal, or one or two types of hydrocarbon gas may be used in combination. In addition, some of the oxidizing gas may be replaced by oxides such as iron ore, manganese ore, nickel ore and chromium ores that are easily reduced by the carbon contained in the molten steel. In this case, the replacement rate is 30 wt% of the total oxygen content in order to minimize the temperature drop of the molten steel.
Should be less than or equal to%.

At this time, the size of these solid oxides added to the molten steel is preferably in the range of 0.1 to 30 mm in order to secure the yield and reactivity. The present invention applies to current vacuum refining equipment such as DH, RH, VOD,
It can also be applied when denitrifying molten steel with equipment such as VAD.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 The denitrification treatment of molten steel having a pressure in the vacuum chamber of 20 mmHg or less, a temperature of 1650 ° C. and a weight of 250 tons was carried out in a vacuum degassing furnace as shown in FIGS. 2 (a) and 2 (b). Carried out.
[C] concentration is in the range of 0.015-0.50 wt%,
The [O] concentration is in the range of 0.01 to 0.03 wt%.

The oxidizing gas used is oxygen gas, carbon dioxide alone, a mixed gas of oxygen gas and carbon dioxide (50%), or a mixed gas of oxygen gas and water vapor (20%).

The oxidizing gas was supplied to the molten steel through a nozzle in a vacuum chamber. [C] concentration of molten steel is 0.01
The carbon source supplied to maintain the wt% or more was graphite waste, coke, coal alone or a mixture thereof, and was supplied to the molten steel in the vacuum chamber. Based on the denitrification rate constant k _N ⁰ (1 / wt% · min) when the molten steel is not decarburized, the denitrification rate constant k _N (when denitrifying while decarburizing the molten steel by blowing an oxidizing gas) The relationship between the ratio k _N / k _N ⁰ with 1 / wt% · min) and the decarburization rate Vc [wt% / min] is shown in FIG.

The value of k _N / k _N ⁰ increases as Vc increases. However, when Vc becomes 0.005 [wt% / min] or more, the rate of increase in the value of k _N / k _N ⁰ rapidly increases. There was no difference in this relationship due to the difference in the type of oxidizing gas used and the type of the supplied carbon source, and no difference due to the difference in the vacuum degassing furnace used. Example 2 The pressure in the vacuum chamber was 200 mmHg or less, and the temperature was 1650 ° C.
Fig. 2 (a) shows the denitrification treatment of molten steel with a weight of 250 tons.
And a vacuum degassing furnace as shown in FIG.
[C] concentration is in the range of 0.015-0.50 wt%,
The [O] concentration is in the range of 0.01 to 0.03 wt% and the decarburization rate Vc is in the range of 0.005 to 0.018 wt% / min.

The oxidizing gas used is oxygen gas, carbon dioxide alone, a mixed gas of oxygen gas and carbon dioxide gas (50%), or a mixed gas of oxygen gas and steam (20%). ..

The oxidizing gas was supplied to the molten steel through a nozzle in the vacuum chamber. The concentration of [C] in molten steel is 0.0
The carbon source supplied to keep the content above 1 wt% is graphite scrap,
Coke, coal alone and a mixture thereof were supplied to molten steel in a vacuum chamber. Figure and denitrification rate constant k _N ⁰ the ratio k _N / k _N ⁰ values of the denitrification rate constant k _N under the pressure, the relationship between the vacuum chamber pressure below the vacuum chamber pressure 0.1mmHg 3 shows. The value of k _N / k _N ⁰ begins to drop extremely when the pressure in the vacuum chamber exceeds 70 mmHg.

At this time, FIG. 4 shows the relationship between the molten steel splash amount index W / Wo (hereinafter referred to as the bare metal attachment amount index) associated with decarburization attached to the vacuum chamber and the atmospheric pressure in the vacuum chamber. Wo is the amount of metal adhered when the pressure in the vacuum chamber is 10 mmHg, and W is the amount of metal adhered under each pressure. The bare metal adhesion index extremely increases when the pressure in the vacuum chamber becomes 3 mmHg or less.

Therefore, in the denitrification treatment, the pressure in the vacuum chamber is set to 1 to
By controlling to about 70 mmHg, and preferably in the range of 3 to 30 mmHg, efficient denitrification treatment can be performed without lowering the molten steel yield due to metal adhesion. During this denitrification period, oxygen gas is supplied using the lance installed at the top,
50 to 90% of the generated CO gas was burned. As a result, the bare metal adhesion index was reduced to about half, which was more efficient. At this time, the pressure in the vacuum chamber is 1 to 70 mmHg
Good as a degree.

Example 3 The pressure in the vacuum chamber was 10 mmHg, the temperature was 1650 ° C., and the weight was 2
The denitrification treatment of the molten steel of 50 tons was carried out in the vacuum degassing furnace shown in FIG. As shown in FIG. 6, the carbon source is supplied to the molten steel, the decarburization rate is set to about 0.01 wt% / min, and the [C] concentration is maintained at 0.01 to 0.03 wt% to perform the denitrification reaction. Ultra low nitrogen molten steel can be melted without hindering

At this time, the carbon source is coke or coal,
C ₂ H ₂ was used. When coke or coal was used, it was added to the molten steel in the vacuum tank, and C ₂ H ₂ was blown into the molten steel from the outer tube of the oxygen gas blowing double tube nozzle using a lance. Therefore, in order to generate CO gas bubbles and increase the denitrification reaction without increasing the [O] concentration, it is necessary to secure the [C] concentration of 0.010 wt% or more.

Example 4 The denitrification treatment of molten steel having a pressure in the vacuum chamber of 20 mmHg or less, a temperature of 1650 ° C., and a weight of 250 tons was carried out under reduced pressure degassing as shown in FIGS. 2 (a) and 2 (b). It was carried out in a gas furnace. The decarburization rate Vc is in the range of 0.01 to 0.02 wt% / min. Ratio of denitrification rate constant k _N at each pressure and decarburization rate to denitrification rate constant k _N ^0.01 obtained when [O] concentration is 0.01 wt% at each pressure and decarburization rate k _N The relationship between / k _N ^0.01 and the [O] concentration is shown in FIG. 7.

The value of k _N / k _N ^0.01 is 0.
When the concentration is higher than 030 wt%, the concentration is extremely reduced and it becomes difficult to denitrify. Therefore, in order to efficiently denitrify molten steel, it is preferable to control the [O] concentration to 0.030 wt% or less. Comparative Example As shown in FIG. 5, when supplying molten steel with oxygen gas for decarburization, when the [C] concentration was less than 0.010 wt%,
It can be seen that the [O] concentration increases and the [N] removal reaction does not proceed.

[0026]

According to the present invention, denitrification of molten steel is 10 ppm.
It becomes possible to denitrify to the following extremely low nitrogen concentration, which facilitates the production of extremely low nitrogen steel.

[Brief description of drawings]

[Fig. 1] Appearance of denitrification while decarburizing molten steel by blowing an oxidizing gas, based on the apparent denitrification rate constant k _N ⁰ (1 / wt% · min) when decarburizing molten steel 2 is a diagram showing the relationship between the ratio k _N / k _N ⁰ of the denitrification rate constant k _N (1 / wt% · min) and the decarburization rate Vc [wt% / min].

FIG. 2 is a drawing showing an example of a degassing facility for carrying out the present invention.

FIG. 3 shows the apparent denitrification rate constant k _N ⁰ measured under a vacuum chamber atmosphere pressure of 0.1 mmHg or less and the ratio k _N / k _N ^{0 of} the apparent denitrification rate constant k _N measured under each pressure. 2 is a drawing showing the relationship with the pressure in the vacuum chamber.

FIG. 4 is a diagram showing a relationship between a metal adhesion amount index W / Wo associated with decarburization attached to a vacuum chamber and a pressure in the vacuum chamber.

FIG. 5 is a drawing showing changes over time in [N], [C], and [O] concentrations during denitrification treatment (comparative example).

FIG. 6 is a drawing showing changes over time in [N], [C], and [O] concentrations during denitrification treatment (Example 3).

FIG. 7: Denitrification rate constant k _N at each pressure and each decarburization rate,
Each pressure, each decarburization rate [O] concentration is a drawing-showing the relationship between the ratio k _N / k _N ^0.01 with the denitrification rate constant k _N ^0.01 obtained when the 0.01 wt% and [O] concentration ..

[Explanation of symbols]

 1 Molten Steel 2 Oxidizing Gas Injection Nozzle 3 Vacuum Tank (RH Type Vacuum Tank) 4 Stirring Injection Plug 5 Oxygen Gas Injection Lance 6 Ladle 7 Reflux Gas Injection Nozzle

Claims (4)

[Claims] 1. When performing denitrification of molten steel while introducing a portion of the molten steel into a decompression tank and decarburizing the molten steel, the decarburization rate or the decarburization amount of molten steel converted per unit time is 0.0
05 [wt% / min] or more, the oxygen concentration [O] of the molten steel is controlled to 0.030 wt% or less, and the carbon concentration [C] of the molten steel is maintained at 0.010 wt% or more, while the oxidizing property of the molten steel is increased. A method for smelting ultra-low nitrogen steel characterized by directly blowing gas. 2. The method for melting ultra-low nitrogen steel according to claim 1, wherein the atmospheric pressure of the vacuum chamber in contact with the free surface of molten steel is controlled to 70 mmHg or less. 3. The method according to claim 1, wherein in order to maintain the [C] concentration of the molten steel at 0.010 wt% or more, coke, coal or hydrocarbon gas is used as a carbon source.
A method for smelting ultra-low nitrogen steel, which comprises continuously or intermittently supplying one or more carbon sources to the molten steel. 4. The method according to claim 1, wherein CO gas generated by decarburization during the denitrification period is burned by supplying oxygen gas into the vacuum chamber. Manufacturing method.