JP3412924B2 - Refining method of chromium-containing molten steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to refining of molten chromium-containing steel for reducing the amount of expensive Ar gas and efficiently controlling the [N] concentration in the reduction treatment after decarburization of molten chromium-containing steel. Concerning the law.

[0002]

2. Description of the Related Art Conventionally, 11mas such as stainless steel
As a refining method for molten chromium-containing steel containing s% or more of chromium, AOD in which oxygen gas or a mixed gas of oxygen gas (hereinafter simply referred to as oxygen) and an inert gas is blown from below the bath surface
The method is widely used. In the AOD method, when decarburization progresses and [C] concentration in molten steel decreases, [Cr] is easily oxidized. Therefore, as the [C] concentration decreases, Ar gas and N in the blown gas are reduced. _A method of suppressing the oxidation of [Cr] by increasing the ratio of an inert gas such as ₂ gas and decreasing the ratio of oxygen is adopted. However, since the oxidation of [Cr] proceeds even in this method, after decarburization to the target [C] concentration,
In order to recover the oxidized [Cr], a reducing treatment is performed by adding a reducing agent such as Si or Al while blowing only an inert gas.

Chromium-containing molten steel is contained in a large amount [Cr].
Significantly reduces the activity of [N] in molten steel,
The solubility of [N] is high, and when only N ₂ gas is blown as the inert gas, the [N] concentration becomes 0.2 mass% or more. Except for special steel grades, the [N] concentration required for general products is 0.1 mass% or less. Therefore, in the AOD method, expensive Ar gas is used as the inert gas except for the initial stage of decarburization. Needed and was at a financial disadvantage.

As a method of solving such a problem,
JP-A-4-21486 and JP-A-4-2630
The method described in Japanese Patent Publication No. 05 has been proposed. The refining method for these molten chromium-containing steels uses the same refining vessel [C].
A mixed gas of a non-oxidizing gas mainly composed of N ₂ gas and oxygen is used as a blowing gas up to a concentration of 0.2 to 0.05 mass%, and after the [C] concentration falls within this range, 2
The pressure is reduced to 00 to 15 Torr, and N is used as a blowing gas.
Decarburization to the target [C] concentration is performed using only non-oxidizing gas consisting mainly of ₂ gases, and then the injected gas is A
This is a method in which a reducing agent is added and treated in order to recover oxidized [Cr] as r gas. By these methods, it is not necessary to use Ar gas in the decarburization treatment, but it is essential to use Ar gas in the reduction treatment. Also,
In these methods, since the degree of vacuum and the blowing time of Ar gas during the reduction treatment are not specified, the [N] concentration after the reduction treatment varies, and it cannot be said that the refining is efficient.

[0005]

DISCLOSURE OF THE INVENTION The present invention aims at the degree of vacuum during reduction treatment in a refining method for molten chromium-containing steel using vacuum refining after atmospheric refining using the same refining vessel [N].
Providing an efficient refining method that makes it possible to use inexpensive N ₂ gas instead of expensive Ar gas during the reduction process and to easily control the [N] concentration by controlling according to the concentration The purpose is to do.

[0006]

The present invention advantageously solves the above-mentioned problems, and the gist thereof is as follows. (1) In a refining method for molten chromium-containing steel, in which decarburization treatment under atmospheric pressure is performed using the same refining vessel, decarburization and reduction treatment under vacuum is performed, in molten steel per ton of molten gas as a blowing gas of the reduction treatment. Smelting of chromium-containing molten steel, characterized in that N ₂ gas of 0.2 Nm ³ / min or more is blown, and reduction treatment is carried out in accordance with the target [N] concentration at a vacuum degree satisfying the following formula and formula for at least 1 min. Law. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009) ... P ≤ 760 ([N] ² x 10 ^-2a ) +10 ... T: Molten steel temperature (° C) [Ni]: [Ni] concentration in molten steel (mass%) [Cr]: [Cr] concentration in molten steel (mass%) P: Degree of vacuum during reduction treatment (Torr) [N]: Target [ N] concentration (mass%) (2) In the refining method for molten chromium-containing steel, which comprises decarburizing treatment under atmospheric pressure using the same refining vessel, decarburizing and reducing treatment under vacuum, As blowing gas, N ₂ gas of 0.2 Nm ³ / min or more per ton of molten steel and Ar
A mixed gas of gas is blown, and the degree of vacuum satisfying the following formula and formula is adjusted to at least the target [N] concentration and at least 1 m.
A method for refining molten chromium-containing steel, characterized by performing a reduction treatment. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009) ... P≤760 ([N] ² × 10 ^-2a ) {(Q _Ar + Q _N2 ) / Q _Ar } +10 ...... T: Molten steel temperature (° C) [Ni]: [Ni] concentration (mass%) in molten steel [Cr]: [Cr] concentration (mass%) in molten steel P: During reduction treatment Degree of vacuum (Torr) [N]: Target [N] concentration (mass%) Q _Ar : Ar gas blowing flow rate (Nm ³ / min) Q _N2 : N ² gas blowing flow rate (Nm ³ / min) ( 3) The refining method for molten chromium-containing steel as described in the above item (1) or (2), which comprises performing decarburization treatment under atmospheric pressure using the same refining vessel and then performing only reduction treatment under vacuum.

The present invention will be described in detail below. The refining method for molten chromium-containing steel of the present invention is a refining method as illustrated in FIG. The refining gas 5 is blown into the molten chromium-containing steel 4 in the refining vessel 1 through the bottom blowing tuyere 2. Further, the refining container 1 has a detachable exhaust hood 3 and can reduce the pressure to an arbitrary degree of vacuum. In the refining of the method of the present invention,
At the beginning of refining, atmospheric pressure refining is performed without the exhaust hood 3, then the exhaust hood 3 is attached, vacuuming is started, and vacuum refining is performed.

The present invention focuses on the fact that the denitrification rate is high in the reduction treatment in which the concentration of [O] in the molten steel becomes low. When N ₂ gas is blown, the degree of vacuum is adjusted, N ₂ gas and Ar gas are adjusted. Degree of vacuum and N in the blown gas when blowing the mixed gas of
By adjusting the ratio of the _two gases, it is possible to adjust the target [N] concentration required for the product.

FIG. 2 shows the relationship between the vacuum degree and the [N] concentration in the molten steel during the reduction treatment when SUS304 stainless steel is subjected to atmospheric pressure refining using the same refining vessel and then vacuum refining. In addition, using only N ₂ gas as the blowing gas,
It is a value after blowing for 1 min or more at 0.2 Nm ³ / min or more per ton of molten steel in a state where the degree of vacuum is constant. The circles in the figure are the measured values, and the solid line is the line obtained experimentally. It can be seen from FIG. 2 that the [N] concentration decreases as the vacuum degree decreases. For example, to adjust the [N] concentration to 0.05 mass%, the degree of vacuum may be set to about 40 Torr or less. The solid line in the figure is the relation obtained under the conditions of various steel types, molten steel temperature and vacuum degree. a = −1.35− {3280 / (T + 273) −0.75} (0.01 [Ni] −0.045 [Cr] +0.009)… P = 760 ([N] ² × 10 ^−2a ) Fig. 2 and formula, From the formula, molten steel temperature, [N in molten steel
If the vacuum degree is adjusted according to the i] and [Cr] concentrations, N
It was confirmed that even if only _two gases were blown in, it could be controlled to an arbitrary [N] concentration.

In FIG. 3, the ratio of Ar gas in the total gas of the blowing gas and the molten steel [N] in the reduction process when SUS304 stainless steel is smelted under atmospheric pressure using the same smelting vessel and then vacuum smelting is carried out. The relationship of concentration is shown. As a blowing gas, N ₂ alone or a mixed gas of Ar gas and N ₂ gas was used, and the value after blowing for 1 min or more at 0.2 Nm ³ / min or more per ton of molten steel at a constant vacuum degree of 100 Torr. Yes, the circles in the figure are the measured values, and the solid line is the line obtained experimentally. It can be seen from FIG. 3 that the [N] concentration decreases as the ratio of Ar gas in the gas increases, despite the same vacuum degree. The solid line in the figure is the relationship obtained under the conditions of various steel types, molten steel temperature, Ar gas ratio of the blowing gas, and degree of vacuum. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009) ... P = 760 ([N] ² × 10 ^-2a ) {(Q _Ar + Q _N2 ) / Q _Ar } ...... From Fig. 3 and the formula, the molten steel temperature, [N
It was confirmed that the [N] concentration can be controlled arbitrarily by adjusting the degree of vacuum according to the i], [Cr] concentration and the Ar gas ratio of the blown gas.

The equations, the equations, and the relations of the equations show average values, and in consideration of variations in the operation, N
_When only ₂ gases are blown, the reduction degree may be obtained by the following equation and the vacuum degree obtained by the equation, and when a mixed gas of N ₂ gas and Ar gas is blown, the reduction degree may be obtained by the following equation and the vacuum degree obtained by the equation. It was confirmed. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009)… P ≦ 760 ([N] ² × 10 ^-2a ) +10 …… P ≦ 760 ( [N] ² × 10 ^-2a ) {(Q _Ar + Q _N2 ) / Q _Ar } +10 ...... In Fig. 4, the target was obtained when SUS304 stainless steel was subjected to atmospheric pressure refining using the same refining vessel and then vacuum refining. The relationship between the blowing gas flow rate and the target [N] concentration achievement rate in the case where the reduction treatment is performed according to the above equation, the equation, and the degree of vacuum obtained from the equation according to the [N] concentration is shown. It should be noted that N ₂ gas alone or a mixed gas of N ₂ gas and Ar gas was used as the blowing gas and was blown for 1 min or more. The target [N] concentration achievement rate was defined by the following formula.

[0012]

[Equation 1]

As can be seen from FIG. 4, the target [N] concentration achievement rate increases as the blowing gas flow rate increases, and becomes almost 100% when the blowing gas flow rate is 0.2 Nm ³ / min or more per ton of molten steel. Therefore, the flow rate of blown gas must be 0.2 Nm ³ / min or more per ton of molten steel.
In Fig. 5, when SUS304 stainless steel is subjected to atmospheric pressure refining using the same refining vessel and then vacuum refining is performed, reduction treatment is performed according to the above formula, the formula and the degree of vacuum obtained from the formula according to the target [N] concentration. The relationship between the refining time at the degree of vacuum and the target [N] concentration achievement rate in the case of the above is shown. As the blowing gas, N ₂ gas alone or a mixed gas of N ₂ gas and Ar gas was added in total of 0.2 Nm ³ / mi per ton of molten steel.
I blew it over n.

As can be seen from FIG. 5, the target [N] concentration achievement rate increases as the refining time increases, and the refining time is 1 mi.
Almost 100% when n or more. Therefore, at least 1 min is required as the refining time. From the above, according to the method of the present invention, in the refining method of performing atmospheric pressure refining using the same refining vessel and then vacuum refining, the degree of vacuum during the reduction treatment is set according to the target [N] concentration, and the degree of vacuum is set. By blowing N ₂ gas alone or a mixed gas of N ₂ gas and Ar gas below, the target [N] concentration can be controlled, and efficient refining can be performed. Further, in vacuum refining, there are steel types that do not require decarburization refining depending on the type of steel, but in this case, efficient refining can be performed by performing only reduction refining under vacuum.

[0015]

The reaction for removing [N] in molten steel, that is, the denitrification reaction, is represented by the following equation, and the reaction equilibrium constant K is represented by the equation. N = 1 / 2N ₂ (g) K = P _N2 ^1/2 / a _N ...... where P _N2 is the partial pressure (atm) of N ₂ gas in the atmosphere,
a _N is the activity of [N] in the molten steel.

In chromium-containing molten steel, [N] contained in a large amount in the molten steel reduces a _N , so that the solubility of [N] becomes large. To denitrify to the target [N] concentration,
As means for lowering P _N2 , application of vacuum refining and a method of using Ar gas alone as a blowing gas have been performed. Conventionally, molten steel composition in the case of using the Ar gas as the blown gas, the equilibrium between the molten steel temperature and vacuum degree [N] concentration relationships have been variously shown, under vacuum refining N ₂ gas or N
Molten steel composition when a mixed gas of ₂ gas and Ar gas is used,
There is no relationship between the molten steel temperature and the degree of vacuum and the equilibrium [N] concentration.
The present inventors derived the above equations, the equations, and the relationships of the equations from experiments under various conditions. Further, based on this relationship, the formulas, formulas, and formulas, which are the conditions during operation, were obtained.

It is well known that the denitrification reaction becomes faster when the [O] concentration in the molten steel, which is a surface-active element, decreases. Therefore, it is efficient to perform the denitrification treatment by the reduction treatment after the decarburization refining. However, until now, Ar gas has been used as the blowing gas in this case, and the denitrification behavior when using N ₂ gas or a mixed gas of N ₂ gas and Ar gas has not been clear. The inventors of the present invention have found that the denitrification reaction proceeds even when N ₂ gas or a mixed gas of N ₂ gas and Ar gas is used under vacuum, and the reduction treatment is performed at the vacuum degree obtained by the above formula, formula and formula. It has been found that the denitrification reaction up to the target [N] concentration easily proceeds if performed.
Furthermore, suitable conditions for the gas injection flow rate and refining time for reducing the target [N] concentration were found.

According to the method of the present invention, it is not necessary to use expensive Ar gas as a blowing gas even in the decarburization process until the reduction process is started, and as the oxygen diluting gas, N ₂ gas or air such as air can be used. It has become possible to use inexpensive gases including N ₂ gas.

[0019]

[Example] SUS304 stainless steel (8 mass% N
i-18 mass% Cr) and the target [C] concentration is 0.0
5 to 0.07 mass%, and [N] concentration is 0.03 to 0.
The treatment of 60 tonnes of steel requiring a range of 05 mass% was carried out in the embodiment shown in FIG. [C] at the start of decarburization
All the concentrations were 1.5 mass%, and refining was performed using oxygen or a mixed gas of oxygen and an inert gas until the vacuum refining was started, and in the vacuum refining, refining was performed using only an inert gas. The decarburization treatment up to the target [C] concentration was performed both at atmospheric pressure and in vacuum refining. In the reduction treatment, Fe-Si was used as the reducing agent, and the amount of the reducing agent required to recover the [Cr] oxidized by the decarburization treatment was calculated. This amount also includes the [Si] concentration required for the product. Fe-Si was added thereto to blow an inert gas under vacuum. The reduction treatment was performed for 5 minutes. After that, the pressure was returned to atmospheric pressure and the steel was tapped in a ladle.

Table 1 shows examples of vacuum degree during decarburization and refining conditions during reduction. The examples of the present invention were carried out so as to satisfy the above-mentioned conditions. Comparative example N
o. No. 7 is an example of performing reduction treatment under atmospheric pressure, N of comparative example
o. 8, No. Reference numeral 9 is a conventional example.
Nos. Of Comparative Examples and Examples carried out according to the methods described in Japanese Patent Laid-Open No. Hei 4-263005. In No. 10 of the comparative example, the ultimate vacuum during processing is outside the conditions of the present invention. No. 11 of the comparative example is the case where the gas injection flow rate is outside the conditions of the present invention.
12 is an example where the refining time under the ultimate vacuum is outside the conditions of the present invention.

The results of implementation are shown in Table 2. The values of the basic unit of Ar gas and the refining cost in the table are shown in No. It is a value obtained by proportionally converting the result of 1 as 100. In the present invention, the reached [N] concentration is the target [N] concentration of 0.03 to 0.05 mass.
Achieves s%, and is stable at a low level in terms of basic unit of Ar gas and refining cost. On the other hand, in the comparative example, the target [N]
In some cases, the concentration is not achieved, and the unit cost of Ar gas and refining costs are high.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

According to the method of the present invention, in the vacuum refining after the atmospheric refining using the same refining vessel for molten chromium-containing steel, the reduction treatment is performed by using N ₂ gas or a mixed gas of N ₂ gas and Ar gas. The refining conditions can be arbitrarily selected according to the target [N] concentration, and the component deviation can be reduced. In addition, it is possible to significantly reduce the basic unit of Ar gas, and it is possible to significantly reduce the refining cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a refining vessel according to an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the degree of vacuum and N concentration by blowing N ₂ gas in the method of the present invention.

FIG. 3 is a diagram showing a relationship between an Ar / (N ₂ + Ar) ratio and [N] concentration in a gas obtained by blowing a mixed gas of N ₂ gas and Ar gas in the method of the present invention.

FIG. 4 is a diagram showing the reason for limiting the lower limit of the blown gas flow rate in the method of the present invention.

FIG. 5 is a diagram showing the reason for limiting the lower limit of the refining time in the method of the present invention.

[Explanation of symbols]

1 refining container 2 Bottom blown tuyere 3 exhaust hood 4 Molten steel 5 gas

Front page continuation (72) Inventor, Hiroshi Iwasaki, 3434, Shimada, Hikari-shi, Yamaguchi Prefecture, Nippon Steel Co., Ltd., Hikari Steel Works (72) Inventor, Hiroaki Morishige, 3434, Shimada, Hikari-shi, Yamaguchi Prefecture, Nippon Steel Corporation (56) References JP-A-8-85811 (JP, A) JP-A-7-188727 (JP, A) JP-A-4-263005 (JP, A) JP-A-4-214816 (JP , A) JP 58-197211 (JP, A) JP 51-12320 (JP, A) JP 2-93016 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) C21C 7/00 C21C 7/068 C21C 7/10

Claims (3)

(57) [Claims] 1. A refining method for chromium-containing molten steel, which comprises decarburizing treatment under atmospheric pressure using the same refining vessel, and then performing decarburizing and reducing treatment under vacuum, in which molten steel is used as a blowing gas for the reducing treatment. Molten chrome-containing steel characterized by injecting N ₂ gas of 0.2 Nm ³ / min or more per ton and subjecting to a target [N] concentration and performing a reduction treatment for at least 1 min at a vacuum degree that satisfies the following formula and formula: Refining method. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009) ... P ≤ 760 ([N] ² x 10 ^-2a ) +10 ... T: Molten steel temperature (° C) [Ni]: [Ni] concentration in molten steel (mass%) [Cr]: [Cr] concentration in molten steel (mass%) P: Degree of vacuum during reduction treatment (Torr) [N]: Target [ N] concentration (mass%) 2. In a refining method for molten chromium-containing steel, which comprises decarburizing treatment under atmospheric pressure using the same refining vessel, followed by decarburizing and reducing treatment under vacuum, molten steel is used as a blowing gas for the reducing treatment. A mixed gas of N ₂ gas and Ar gas of 0.2 Nm ³ / min or more per ton should be blown, and the reduction treatment should be performed for at least 1 min at a vacuum degree that satisfies the following formula and formula according to the target [N] concentration. Characterizing method for refining molten chromium-containing steel. a = -1.35- {3280 / (T + 273) -0.75} (0.01 [Ni] -0.045 [Cr] +0.009) ... P≤760 ([N] ² × 10 ^-2a ) {(Q _Ar + Q _N2 ) / Q _Ar } +10 ...... T: Molten steel temperature (° C) [Ni]: [Ni] concentration (mass%) in molten steel [Cr]: [Cr] concentration (mass%) in molten steel P: During reduction treatment Degree of vacuum (Torr) [N]: Target [N] concentration (mass%) Q _Ar : Flow rate of Ar gas (Nm ³ / min) Q _N2 : Flow rate of N ² gas (Nm ³ / min) 3. The refining method for molten chromium-containing steel according to claim 1, wherein the decarburization treatment is carried out under the atmospheric pressure using the same refining vessel, and only the reduction treatment is performed under vacuum.