JP3225747B2 - Vacuum degassing of molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining molten steel using an RH vacuum degassing apparatus, which suppresses a temperature drop of molten steel and simultaneously removes and reduces metal ingot adhering to walls in a vacuum chamber. It relates to a method for performing gas decarburization.

[0002]

2. Description of the Related Art In a method for degassing and decarburizing molten steel by an RH vacuum degassing apparatus, the following method is known as a method using oxygen gas as a refining gas.

JP-A-53-81416 and JP-A-59-8970
No. 8 states that deoxidized molten steel is added to Al in a vacuum chamber or ladle.
A method is disclosed in which a heating agent such as Si is added and then oxygen gas is blown upward to raise the temperature of the molten steel.

Japanese Patent Application Laid-Open No. 2-77518 discloses a method in which oxygen gas is blown upward onto undeoxidized molten steel in order to raise the temperature of molten steel, and CO in exhaust gas is burned in a vacuum chamber to generate heat. I have.

[0005] By adopting such a method of raising the temperature of molten steel, it is not necessary to raise the temperature of the molten steel in the converter, which is a pre-process of RH processing, and the temperature drop during RH processing can be reduced by RH vacuum degassing. It became possible to compensate by the device. For this reason, temporal and thermal overloads in the converter were eliminated.

However, when expensive raw materials such as Al and Si are used as the exothermic agent, the cost is increased, and there is a concern that Al ₂ O ₃ and SiO ₂ generated after the supply of oxygen gas adversely affect the steel quality. .

As a method of accelerating the decarburization of molten steel, for example, RH which increases oxygen in steel by blowing oxygen gas over undeoxidized molten steel in iron and steel, vol. 64 (1978), S635 -OB
The law has been reported.

[0008]

In addition to the above method of blowing oxygen gas during RH decarburization as disclosed in Japanese Patent Application Laid-Open No. 2-77518, RH decarburization is not only performed in the case of ordinary RH decarburization.
During vacuum decarburization, the decarburized product gas (CO gas) is released into the vacuum from the surface of the molten steel, so that violent bubble burst-like fine molten steel particles are generated. In the process of randomly scattering toward the provided evacuation duct, it adheres as metal in the entire vacuum chamber. For this reason, generally, the RH treatment is periodically interrupted, or the RH vacuum tank is suspended and cooled, and a metal cutting operation in the vacuum tank using a flame burner is performed.

[0009] The adhesion of the bullion as described above hinders the continuous use of the RH vacuum chamber, and requires a great deal of man-hour for the slab cutting operation. Development of a gas decarburization method is desired.

It is an object of the present invention not only to suppress the temperature drop of molten steel due to oxygen gas top blowing, but also to simultaneously dissolve and reduce the deposited metal substantially uniformly from the upper part to the lower part of the inner wall of the tank. RH vacuum degassing and decarburizing method.

[0011]

The gist of the present invention lies in the following vacuum degassing and decarburizing method for molten steel.

When degassing and decarburizing molten steel using an RH vacuum degassing apparatus, an upper blowing lance that can be moved up and down from the upper part of the vacuum tank into the vacuum tank is inserted, and the molten steel surface in the vacuum tank is inserted from this lance. Oxygen gas is blown under the condition that satisfies the following formula,
A method for vacuum degassing and decarburizing molten steel, characterized in that the CO gas in the exhaust gas is combusted in the tank, thereby suppressing a drop in the temperature of the molten steel and simultaneously dissolving the metal that has adhered to the walls in the vacuum tank. .

L = (α · D ^0.5 · Q) / [(d _t ) ^0.5 · P ^0.1 · W] where L: distance between lances (m) D: nozzle throat expansion rate (nozzle Q: Oxygen gas flow rate (Nm ³ / min) _dt : Nozzle throat diameter (m) P: Degree of vacuum in the vacuum tank (Torr) (1 ≦ P ≦ 100) W: Molten steel reflux (Ton / min) α: proportionality constant (1.5 ≦ α ≦ 3) The above “nozzle throat” refers to a throttle portion (minimum inner diameter portion) in the case of a so-called Laval nozzle. In the case of a multi-hole nozzle, D may be obtained for one nozzle hole.

The present invention has the following features.

[0015] The oxygen in the exhaust gas is increased by the top-blown oxygen gas.
The gas is subjected to secondary combustion in the tank, and the generated heat is given to molten steel to suppress a temperature drop.

At the same time, the generated heat is applied to the refractory to such an extent that the refractory of the tank wall is not damaged, and the metal adhered from the upper part (including the ceiling part) to the lower part of the tank wall is substantially uniform. Dissolves and reduces.

The present inventors examined various operating conditions,
In order to satisfy the above and at the same time, it is necessary to control the distance between the lance surfaces to a certain appropriate range, and furthermore, the appropriate distance is the degree of vacuum in the vacuum chamber, the amount of molten steel reflux,
It was found that it was determined by the oxygen gas flow rate and the shape of the lance nozzle.

[0018]

FIG. 1 shows an example of an RH vacuum degassing and decarburizing apparatus to which the method of the present invention is applied.

FIG. 1 is a longitudinal sectional view of an RH vacuum degassing and decarburizing apparatus having an upper blowing lance. In a molten steel 6 accommodated in a ladle 3, two immersion tubes 2 provided at the lower part of the vacuum chamber 1
(2a is an ascending pipe, 2b is a descending pipe), the inside of the vacuum vessel 1 is evacuated, molten steel 6 is introduced into the vacuum vessel 1, and Ar gas is introduced from the reflux gas injection tuyere 4 connected to the ascending pipe 2a. The gas is blown, the molten steel 6 is raised through the riser 2a based on the principle of gas lift, and is vacuum degassed and decarburized while being returned to the ladle 3 again through the descender 2b. In this state,
The upper blowing lance 5 is lowered from the upper part (the ceiling part) of the vacuum tank 1 into the tank, and oxygen gas is blown upward.

In the method of the present invention, when the oxygen gas is not blown upward, the upper blow lance 5 is raised and retracted in the vacuum chamber 1 to a position which is not affected by the molten steel splash.

It is desirable that the upper lance 5 has a water-cooled structure.

When undeoxidized molten steel is treated by the apparatus shown in FIG. 1, C and O in the molten steel react with each other in accordance with the following equation to generate CO gas.

C + O → CO ··········· Upper lance 5
When oxygen gas is blown from the CO, the CO gas reacts with the oxygen gas according to the following formula to generate CO ₂ gas, and at that time, heat of combustion is generated. This heat is used to control the temperature drop of the molten steel.

2CO + O ₂ → 2CO ₂ ... Further, at the same time, in order to substantially uniformly dissolve and reduce the adhered metal in the vacuum chamber 1 from the upper part to the lower part of the tank inner wall, the following condition is satisfied. There is a need.

L = (α · D ^0.5 · Q) / [(d _t ) ^0.5 · P ^0.1 · W] where L: distance between lance surfaces (m) D: nozzle throat expansion rate (nozzle Q: Oxygen gas flow rate (Nm ³ / min) _dt : Nozzle throat diameter (m) P: Degree of vacuum in the vacuum tank (Torr) (1 ≦ P ≦ 100) W: Molten steel reflux (Ton / min) α: proportionality constant (1.5 ≦ α ≦ 3) If necessary, an alloy element or the like is added to molten steel in the vacuum chamber 1 through the ferromagnetic iron inlet 7. Such an operation is performed for a predetermined time to perform degassing and decarburization of molten steel.

Here, α in the formula is a proportionality constant determined to give the combustion heat generated by the formula to the molten steel and the vacuum vessel wall at an optimal ratio. That is, the formula is an empirical formula.

If α is less than 1.5 or more than 3.0, the distance L between the lance surfaces is not affected even if other conditions are appropriate.
Is too small or too large, and an appropriate distance L between the lance surfaces cannot be obtained.

The range of the vacuum degree P in the tank is from 1 to 100 Torr.
The decarburization treatment to 50 ppm is within the range of the degree of vacuum that can be achieved in a treatment time of about 10 to 30 min.

Next, a test to which the above equation is applied will be described with reference to FIGS. The test conditions are as follows.

Amount of molten steel treated by RH device: 160 tons C before treatment: 300 to 500 ppm Molten steel temperature before treatment: 1650 ° C. Nozzle: Single hole (throat diameter _dt is 20 mm, throat expansion rate D is 1.5) Vacuum degree P in the tank: 2 to 50 Torr Molten steel reflux amount W: 80 Ton / min Oxygen gas flow rate Q: 25 Nm ³ / min From the above conditions, the preferred distance L between the lance surfaces is L when the vacuum degree P = 2 Torr. = 3.0m (α = 1.5) 〜6.0m (
α = 3) When the degree of vacuum P = 50 Torr, L = 2.2m (α = 1.5) to 4.4m (
α = 3), but in this test, the distance L between the lance surfaces was fixed at 4 m.

The treatment was carried out under the condition that oxygen gas was not blown upward and under the above-mentioned appropriate conditions, and the degree of drop in molten steel temperature was compared. Further, a treatment test was performed under an appropriate condition of the distance L between the lances, and the condition of the metal adhering to the wall in the vacuum chamber was compared under three types of conditions.

FIG. 2 shows the temperature drop of the molten steel before and after the number of times of continuous use of the vacuum tank after the pause when oxygen gas was blown upward at 25 Nm ³ / min during the initial 10 minutes of the RH decarburization treatment. It is a figure showing (ΔT). As shown in the figure, under the proper conditions determined by the method of the present invention, an effect of suppressing the temperature drop of about 5 to 10 ° C. was obtained as compared with the case where the oxygen gas was not blown up.

FIG. 3 is a longitudinal sectional view of the vacuum tank showing the state of metal adhesion in the vacuum tank. FIG. 3 (a) shows no oxygen gas blowing, FIG. 3 (b) shows oxygen gas blowing under appropriate conditions, and FIG. 3 (c).
Indicates a case where oxygen gas is blown out of proper conditions.

As shown in FIG. 3A, without blowing oxygen gas upward, the base metal 8 adheres thickly from the upper part to the lower part in the vacuum tank, and cannot be reduced during operation.

When the distance L between the lance surfaces is smaller than the value determined by the equation, the splash from the molten steel adheres to the lance, the lance is melted, and as shown in FIG. It is not possible to melt or reduce the metal 8 attached to the surface of the vacuum chamber. On the contrary, heat is excessively applied to the lower portion of the vacuum chamber, and the refractory in the vicinity is also melted. Further, oxygen gas is mainly supplied into molten steel and is not effectively used for combustion of CO gas in exhaust gas.

On the other hand, when the distance L between the lance surfaces is made larger than the value determined by the equation, the combustion of CO gas occurs in the upper portion of the vacuum chamber, the efficiency of heating the combustion heat to the molten steel decreases, and the vacuum The adhered metal in the lower part of the tank cannot be removed, and conversely, too much heat is applied to the upper part of the vacuum tank, so that the refractory in the vicinity also melts.

The oxygen gas flow rate Q is determined by the required degree of temperature rise of the molten steel. For example, according to the above-mentioned results, the amount required for raising the temperature by about 5 to 15 ° C. is generally about 10 to 30 ° C.
Nm is a ^{3 / min (0.5~2Nm 3 / ton} ).

The oxygen gas supply timing is effective when the amount of exhaust gas generated is large and the CO gas concentration in the exhaust gas is high because the amount of secondary combustion by the top-blown oxygen gas increases. Oxygen gas supply time is specifically, when C in the molten steel at the initial stage of decarburization is 300 to 500 ppm, 0 to 15 minutes after the start of the vacuum treatment,
Desirably, 0 to 10 minutes is appropriate.

Secondary combustion rate of exhaust gas [CO ₂ / (CO + C)
O _2)] × 100% in the case of no blowing on oxygen gas, but was increased even about 10%, under the conditions stipulated in the present invention, the 30% to 100%.

The lance nozzle is not limited to a single hole, and three holes, four holes, and the like can be similarly applied according to the capacity of the vacuum chamber. At this time, the nozzle throat enlargement ratio (nozzle outlet diameter / throat diameter) D may be obtained for one nozzle hole.

[0041]

【Example】

[Example of the present invention] Before treatment using a 170 ton RH apparatus, C: 300 to
When processing 500 ppm of molten steel, a water-cooled top blowing lance was inserted into the vacuum chamber, oxygen gas was blown in under the conditions shown in Table 1, and the temperature rise of the molten steel and the state of adhesion of the metal in the tank were investigated. Conditions other than the display are as follows.

Nozzle: single hole (throat diameter _dt is 20 mm, throat expansion ratio D is 2.0) Proportional constant α: 1.5 to 3 Molten steel temperature before treatment: 1650 ° C. The results are also shown in Table 1. The secondary combustion rate is 50-80%
Was in the range.

[0043]

[Table 1]

As shown in Table 1, by controlling the distance L between the lance surfaces within a range defined by the formula,
The temperature of the molten steel was raised by about 10 ° C, and at the same time, the adhesion of metal in the vacuum chamber was reduced uniformly.

The effect of increasing the molten steel temperature is more remarkable as the distance L between the lance surfaces becomes smaller. When the above L is set near the lower limit of the calculated value, it is somewhat difficult to remove the metal at the top of the tank. Conversely, when L is set near the upper limit of the calculated value, the metal at the bottom of the tank is reduced. Removal was somewhat difficult.

[Comparative Example] A process was performed under the conditions shown in Table 2 except that the distance L between the lance surfaces was out of the range defined by the formula. Other conditions were the same as those of the above-described example of the present invention. The results are shown in Table 2.

[0047]

[Table 2]

As can be seen from Table 2, when the distance L between the lance surfaces is out of the range defined by the equation, the effect of increasing the temperature of the molten steel is reduced or the continuation of the operation becomes difficult due to the lance melting. Was.

[0049]

According to the present invention, the degassing and decarburization of molten steel can be performed while suppressing the temperature drop of molten steel and at the same time uniformly dissolving and reducing the deposited metal in the RH vacuum tank from the upper part to the lower part of the inner wall of the tank. It is possible.

[Brief description of the drawings]

FIG. 1 is a vertical sectional view showing an example of an RH vacuum degassing and decarburizing apparatus to which the method of the present invention is applied.

FIG. 2 is a diagram showing an example of the effect of suppressing the temperature drop of molten steel by the method of the present invention.

FIG. 3 is a vertical cross-sectional view of the vacuum tank showing a state of metal adhesion in the vacuum tank in the embodiment. (a) shows no oxygen gas blowing,
(b) shows a case where oxygen gas is blown up under appropriate conditions, and (c) shows a case where oxygen gas is blown up outside appropriate conditions.

[Explanation of symbols]

1: vacuum tank, 2: immersion pipe, 2a: ascending pipe, 2b: descending pipe, 3: ladle, 4: reflux gas injection tuyere, 5: water cooling top blowing lance, 6: molten steel, 7: alloy iron inlet ,
8: bullion

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C21C 7/10 C21C 7/068

Claims (1)

(57) [Claims] 1. When degassing and decarburizing molten steel using an RH vacuum degassing device, an upper blowing lance which can be moved up and down from the upper part of the vacuum tank into the vacuum tank is inserted. Oxygen gas is blown into the molten steel surface under the conditions that satisfy the following formula,
A method for vacuum degassing and decarburizing molten steel, characterized in that the CO gas in the exhaust gas is combusted in the tank, thereby suppressing a drop in the temperature of the molten steel and simultaneously dissolving the metal that has adhered to the walls in the vacuum tank. . L = (α · D ^0.5 · Q) / [(d _t ) ^0.5 · P ^0.1 · W] where L: distance between lances (m) D: nozzle throat expansion rate (nozzle outlet diameter Q: Oxygen gas flow rate (Nm ³ / min) d _t : Nozzle throat diameter (m) P: Degree of vacuum in vacuum chamber (Torr) (1 ≦ P ≦ 100) W: Recirculation amount of molten steel (Ton / min) α: proportionality constant (1.5 ≦ α ≦ 3)