JPH06330140A - Treatment for desulfurizing molten steel - Google Patents

Abstract

PURPOSE:To raise molten steel temp. for compensating the lowering of the molten steel temp. caused by blowing a desulfurizing agent onto the molten steel during RH degassing treatment from a top-blowing lance. CONSTITUTION:An excess Al for deoxidation over the aimed quantity is added beforehand into the molten steel 4 in a ladle 3 and on the other hand, after executing desulfurizing treatment by blowing the desulfurizing agent 7 onto the molten steel 4 in the degassing vessel 1 from the top-blowing lance 2, oxygen is blown from the top-blowing lance 2 to oxidize Al in the molten steel 4 and the temp. of the molten steel 4 is raised to compensate the lowering of the molten steel temp. caused by the addition of the desulfurizing agent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desulfurizing molten steel by spraying a powdered desulfurizing agent together with a carrier gas onto the molten steel in an RH degassing tank.

[0002]

The following methods are generally known as desulfurization treatment methods for molten steel by RH degassing. (1) Method of adding flux from the hopper on the RH tank {iron and steel, 70 (1984) -S979}. (2) Method of injecting flux from the injection port directly under the RH dip tube {Iron and Steel, 72 (1986) -S263, S1108
}. (3) A method of blowing flux from a nozzle provided in the RH lower tank (JP-A-63-114918). (4) A method of spraying a desulfurizing agent together with a carrier gas from an upper blowing lance in the RH degassing tank.

[0003]

With the above-mentioned prior art alone, since the temperature drop of the molten steel due to the flux blowing as a desulfurizing agent is severe, the amount of flux blowing is often limited due to temperature restrictions, and a desired desulfurization is obtained. It is not uncommon to stop the desulfurization process before. Also, when designing the temperature in consideration of the temperature drop due to flux blowing, the temperature of tapping steel from the converter inevitably rises, which causes an increase in converter refractory cost and a decrease in converter dephosphorization ability. Will result.

An object of the present invention is to provide a method for desulfurizing molten steel capable of solving the above-mentioned problems of the prior art.

[0005]

The present invention for achieving the above object provides a method for desulfurizing molten steel by spraying a desulfurizing agent together with a carrier gas from an upper blowing lance onto molten steel in an RH degassing tank. Before the desulfurization treatment or while the desulfurization treatment, the molten steel contains a deoxidizing element in excess of the target, and after the desulfurization treatment by spraying the desulfurizing agent to the molten steel,
By blowing oxygen into the RH degassing tank to oxidize the deoxidizing element in the molten steel, the temperature drop of the molten steel due to the blowing of the desulfurizing agent during the desulfurization treatment is T ₁ , and the temperature of the molten steel due to the oxidation of the deoxidizing element after the desulfurization treatment. A method for desulfurizing molten steel, characterized in that T ₂ ≧ T ₁ when the amount of increase is T ₂ .

[0006]

In the present invention, a deoxidizing element such as Al is added to molten steel in excess of the target component before or during desulfurization treatment, and after desulfurization treatment by blowing a desulfurizing agent to obtain a predetermined S concentration, The temperature is adjusted by supplying oxygen to oxidize and burn a deoxidizing element such as Al. The excess addition of the deoxidizing element such as Al may be performed at the time of tapping the converter or by mixing the deoxidizing element with the desulfurizing agent during the desulfurization treatment.

As a method of supplying oxygen, a method of mixing it into the reflux gas from the lower part of the dip tube was tried, but the refractory erosion due to the large amount of FeO produced by the oxidation of iron is severe and practically difficult. In the present invention, a deoxidizing element such as Al is excessively added to a target component of molten steel to perform desulfurization treatment, and then the deoxidizer is burned by blowing oxygen, so that the molten steel temperature and the component can be easily adjusted. Become.

Generally, the desulfurization reaction of molten steel proceeds according to the following equation. S + (O ²⁻ ) = (S ²⁻ ) + O (1) In order to promote the progress of the desulfurization reaction from the reaction formula (1), O in the steel
It is necessary to decrease the value of (O ²⁻ ) in the slag. O of Al-killed steel, which requires extremely low sulfur, is generally defined by the Al-Al ₂ O ₃ equilibrium.

Al ₂ O ₃ = 2 Al +3 O (2) According to Gakshin, the 19th Committee of Steelmaking, and the recommended equilibrium value of the steelmaking reaction, the equilibrium constant K of the reaction equation (2) is expressed by the equation (3). It log K = log a _Al ² · a ₀ ³ = −64000 / T + 20.57 (3) Here, a _Al : Al activity in molten steel a ₀ : Oxygen activity in molten steel T: Temperature (° K) From the equation (3), the oxygen activity a ₀ in the molten steel can be expressed as follows.

Log a ₀ = (− 64000 / T + 20.57 −log a _Al ² ) / 3 (4) From this equation (4), the T (temperature) is lowered in order to lower the oxygen activity a _{0 of the} molten steel. It can be said that it is necessary to increase the Al activity a _Al . Figure 2 shows CaO-saturated CaO-
The relationship between [Al] and [S] in molten iron in equilibrium with Al ₂ O ₃ system slag is shown. As shown in FIG. 1, [S] decreases as the temperature decreases and [Al] increases. Therefore, it can be seen that the present invention is effective in promoting the desulfurization reaction.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an RH degassing apparatus used in the present invention. An upper blow lance 2 penetrating the ceiling of the RH degassing tank 1 is suspended in the tank, and the molten steel 4 in the ladle 3 is depressurized.
In the degassing tank 1, one dipping pipe 5A is raised, and the other dipping pipe 5B is processed in the degassing tank and is lowered to be returned to the ladle 4.

In the present invention, the powdery desulfurizing agent 7 in the dispenser 6 is supplied to the upper blowing lance 2 through the switching valve 8 together with Ar gas which is a carrier gas, and the upper blowing lance 2 is supplied.
From the above, the molten steel 4 in the RH degassing tank 1 is sprayed for desulfurization, and by opening and closing the switching valve 9, oxygen can be supplied to the upper blowing lance 2.

The desulfurization treatment procedure of 250 tons of molten steel by the method of the present invention using the RH degassing apparatus shown in FIG. 1 will be described. Molten steel 4 was added to the molten steel 4 in the ladle 3 of 250 tons discharged from the converter in excess of the target component as a deoxidizing element.
In excess of [Al]. The molten steel components before degassing are C = 0.07%, Si = 0.20%, Mn = 1.25%, P = 0.007
%, S = 0.0034%, Al = 0.102%, the molten steel temperature is 16
It is 00 ° C.

FIG. 3 shows the transition of molten steel temperature [S] during RH degassing of molten steel. When the initial mixed slag with stable reflux comes out (about 2 minutes after the start of reflux in FIG. 3), the upper blowing lance 2 is lowered and stopped at a height of 2 to 3 m from the bottom 1A of the degassing tank 1. Then, CaO is used as the desulfurizing agent 7 from the upper blowing lance 2.
Blow powder. The blowing speed at that time is 0.5 kg / mi
At n · t, the flow rate of Ar gas as carrier gas is 0.02 Nm ³
/ Min · t. As shown in Fig. 3, the S concentration in the molten steel decreased during the CaO powder injection due to the progress of the desulfurization reaction.
At an amount of 6.5 kg / t, [S] = 10 ppm, so about 15 minutes after starting the reflux, the blowing was finished. At that time, the molten steel temperature was 1560 ° C and [Al] was 0.085%.

After the CaO powder is blown in, the upper blow lance 2
Was raised by 1.5 m from the blowing position and stopped, and oxygen was blown from the same upper blowing lance 2 by switching the switching valves 8 and 9. The oxygen flow rate is 0.14 Nm ³ / min · t. O ₂ blowing was completed 5 minutes after starting the oxygen blowing, and the upper blowing lance 2 was raised. Due to the [Al] oxidation in the molten steel, the molten steel temperature became 1582 ° C and [Al] became 0.025%. Then, the mixture was refluxed for 5 minutes to complete the degassing treatment of the molten steel. [S] at the end of degassing was 11 ppm.

FIG. 4 shows the relationship between the oxygen activity a _O (calculated value) in molten steel and the desulfurization rate of molten steel in the method of the present invention and the conventional method in which Al is added according to the target component and a desulfurizing agent is added from the top blowing lance. Indicates. Here, desulfurization rate = {(before treatment [S]
-After treatment [S]) / Before treatment [S]} × 100 As shown in FIG. 4, according to the method of the present invention, the oxygen activity of the molten steel can be reduced as compared with the conventional method, so that the desulfurization rate is significantly increased. You can see that it can be improved.

[0017]

As described above, according to the present invention, the deoxidizing element such as Al is added to the molten steel in excess of the target, and the desulfurization treatment can be performed in the low temperature range. _O
And the desulfurization reaction is promoted. Moreover, since the desulfurizing agent blowing and the oxygen blowing into the molten steel are combined as one process, the decrease in the molten steel temperature due to the desulfurizing agent blowing can be compensated by the temperature increase of the molten steel due to the oxidation of the deoxidizing element such as Al. it can. Furthermore, by controlling the tapping temperature, it is possible to extend the life of the refractory of the converter and improve the P removal capability.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an RH degassing apparatus used in the method of the present invention.

FIG. 2 is a graph showing the relationship between [Al], [S] in molten steel and molten steel temperature.

FIG. 3 is a graph showing changes in S concentration and temperature of molten steel by the RH treatment of the present invention.

FIG. 4 is a graph showing the relationship between the oxygen activity a _O and the desulfurization rate according to the method of the present invention and the conventional method.

[Explanation of symbols]

 1 RH degassing tank 2 Top blowing lance 3 Ladle 4 Molten steel 5 Immersion pipe 6 Dispenser 7 Desulfurizing agent 8 Switching valve 9 Switching valve

Claims (1)

[Claims] 1. A method of desulfurizing molten steel by spraying a powdered desulfurizing agent together with a carrier gas onto the molten steel in an RH degassing tank, wherein the molten steel is desulfurized before or during desulfurization. Also contains an excessive amount of deoxidizing element, but after the desulfurization treatment by spraying the desulfurizing agent to the molten steel, oxygen is blown into the RH degassing tank to oxidize the deoxidizing element in the molten steel, thereby Assuming that T ₁ is the temperature drop of the molten steel due to the blowing of the desulfurizing agent and T ₂ is the temperature rise of the molten steel due to the oxidation of the deoxidizing element after the desulfurization treatment, T ₂
A method for desulfurizing molten steel, characterized in that ≧ T ₁ .