JP2940358B2 - Melting method for clean 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 smelting clean steel in the RH method.

[0002]

2. Description of the Related Art In recent years, in high-grade steels mainly for linepipe materials, the demands on the usage characteristics of the steel materials have been remarkably strict. To meet these demands, extremely low sulfur, Ca
It is becoming indispensable to melt steel that satisfies the conditions of addition and extremely low hydrogen. Generally, desulfurization of molten iron is divided into hot metal desulfurization performed in a hot metal stage and molten steel desulfurization performed in a molten steel stage after a converter process. In recent years, with the increasing demand for higher purity of molten steel, hot metal desulfurization alone is not sufficient, and molten steel desulfurization has become an essential process. In particular, for the production of ultra low sulfur steel with [S] ≦ 0.001%, the development of a more efficient molten steel desulfurization method is strongly required.

Conventional methods for desulfurizing molten steel include a method of injecting a desulfurizing agent in a ladle and a method of stirring the molten steel after adding the desulfurizing agent. There were problems such as a large pickup. In order to solve these problems, a method of adding a desulfurizing agent during RH vacuum processing has been developed. Of these, the simplest method is to add a desulfurizing agent from the ferromagnetic iron addition port provided in the middle of the vacuum tank.To prevent the desulfurizing agent from being sucked into the exhaust system, the desulfurizing agent is used. Has a disadvantage that it is disadvantageous in terms of reaction efficiency.

As a further developed method, a method of injecting a desulfurizing agent together with a carrier gas from a nozzle immersed in molten steel in a vacuum chamber (Japanese Patent Application Laid-Open No. 61-130413, etc.), blowing powder into a ladle A method of injecting a lance and injecting a desulfurizing agent together with a carrier gas toward an immersion pipe (ascending pipe) (JP-A-58-37112, JP-A-62-1963)
No. 17) has been proposed. However, JP-A-61-1304
No. 13, JP-A-58-37112, and the methods disclosed in JP-A-62-196317 disclose the gas so that molten steel does not enter from the powder injection port even while the desulfurizing agent is not injected. It is necessary to keep it flowing, which is disadvantageous in terms of cost and maintenance of the degree of vacuum.

Therefore, as a method for solving these problems, a technique for accelerating the deoxidation reaction by blowing up a deoxidizing agent from above the molten steel using an upper blowing lance implemented in a VOD furnace.
A method of applying a desulfurizing agent upward by applying to the RH method is considered.
As an example of application to the method, there has been proposed a method in which a powder blowing lance is obliquely installed on the side wall of a vacuum chamber of the RH method and a desulfurizing agent is blown upward (Japanese Patent Application Laid-Open No. 60-184618).

As a Ca addition method for controlling the form of inclusions, there is a method in which a gas is added as a carrier from a lance in which a Ca alloy powder is immersed in molten steel in a ladle and mixed and added to the molten steel. Are known. Further, vacuum degassing has been generally used as a method for ultra-low hydrogenation of high-purity steel, but the boiling point of Ca fluctuates according to a change in pressure. For this reason, since it evaporates easily under a high vacuum, a method has been used in which dehydrogenation is performed before addition of Ca, and then a rise in hydrogen is suppressed using a medium solvent having a small hydrogen content.

[0007]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 60-184618
In the method disclosed in Japanese Patent Laid-Open Publication No. H06-187, the powder blowing lance is obliquely installed on the side wall of the vacuum tank, so that the powder bounces on the molten steel surface due to the angle, and the desulfurizing agent does not effectively enter the molten steel. Has disadvantages. In addition, the method of adding gas as a carrier from a lance in which Ca alloy powder is immersed in molten steel in a ladle uses a lance. In addition to the problem of high temperature, similar to the desulfurization method of injecting a desulfurizing agent into the ladle, there are problems such as a decrease in the temperature of molten steel and a large nitrogen pickup from the atmosphere. Furthermore, the method of suppressing the rise of hydrogen by using a medium solvent having a low hydrogen content after dehydrogenation before adding Ca has the disadvantage that the processing cost is greatly increased because the degree of calcination of the flux is increased. doing.

In addition, these methods which have been conventionally performed have a temperature drop in each step due to a combined treatment in which a single process is combined with the preceding and subsequent steps. Not only must the blow stop temperature in the converter be significantly higher than in the material, but also the processing steps become complicated, resulting in a decrease in productivity due to an extension of the cycle time, which is not suitable for mass production.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned drawbacks of the prior art, to allow a desulfurizing agent to effectively penetrate into molten steel, to reduce Ca alloy powder from lance erosion, and to reduce the temperature of molten steel. It is an object of the present invention to provide a method for smelting clean steel that can be added while suppressing nitrogen pickup from the atmosphere and the atmosphere.

[0010]

Means for Solving the Problems The present inventors have conducted various tests and studies to achieve the above object. As a result, at the time of vacuum degassing at RH, by injecting a desulfurizing agent into molten steel in parallel with it, it is possible to solve the problem of temperature drop of molten steel and nitrogen pickup from atmosphere,
Further, in order to add the Ca alloy during the vacuum degassing treatment, it was clarified that the Ca yield could be improved by adding the Ca alloy after increasing the pressure level to a range that does not stop the reflux of the molten steel, and reached the present invention.

[0011] The present onset Ming, when performing vacuum degassing process by RH method molten steel ended deoxidation, desulfurization process by blowing on the desulfurizing agent with the carrier gas from the lance was inserted vertically from the vacuum chamber upper portion And then increasing the pressure level to a level that does not stop the reflux of the molten steel, and adding calcium alloy particles into the vacuum chamber.

[0012]

In the present invention, when performing a vacuum degassing process by the RH method, a fine desulfurization blown into the molten steel is performed by blowing a desulfurizing agent together with a carrier gas upward from a lance vertically inserted from the upper portion of the vacuum tank. With the molten steel that the agent recirculates, it passes through the downcomer of the dip tube to reach the ladle,
Thereafter, the desulfurizing agent continues the slag-metal reaction while rising through the molten steel to its surface by buoyancy, so that a high desulfurization reaction can be obtained. Generally, in the slag-metal reaction, the better the slag slag property is, the higher the reaction efficiency is. Therefore, the desulfurizing agent used in the present invention has a low melting point,
Those having good slagging properties are more effective. In addition to CaO (quick lime) usually used for desulfurization, those obtained by mixing CaO with CaF ₂ or the like to lower the melting point are effective.

In the blowing of the desulfurizing agent, the distance between the lance and the surface of the molten steel is reduced by using a powder top blowing method in order to sufficiently infiltrate the fine desulfurizing agent into the molten steel.
It is effective to make the atmosphere a high vacuum. To reduce the distance between the lance and the surface of the molten steel, the distance between the lance and the surface of the molten steel can be reduced by lowering the lance, and a fine desulfurizing agent can penetrate into the molten steel as deeply as possible to reduce the scattering loss of the desulfurizing agent. . In addition, by increasing the atmosphere to a high vacuum, the flow velocity increases with the expansion of the carrier gas, so that the resistance received by the powder is also reduced, and the speed of the powder when it reaches the surface of the molten steel is increased. The penetration depth of the powder increases. In the present invention, since the desulfurizing agent is deeply penetrated into the molten steel, the desulfurizing agent is trapped by the molten steel and does not directly contact the refractory, so that the refractory erosion can be reduced.

The lance to be used may be an uncooled lance, but is preferably of a type that can be cooled using cooling water or the like.
If an uncooled lance is used, troubles such as meltdown of the lance and inability to move up and down due to sticking of metal to the lance occur.
Even if the distance between the lance and the surface of the molten steel cannot be maintained accurately, or the lance body is sound, the nozzle at the tip of the lance will be melted, the gas flow rate will change, and the powder blowing direction will change. May change, and the predetermined blowing condition may not be maintained.

After the desulfurizing treatment is performed by blowing a desulfurizing agent together with a carrier gas from a lance vertically inserted from the upper portion of the vacuum tank, the pressure is raised to a pressure level within which the reflux of the molten steel is not stopped, and the calcium alloy particles are removed from the vacuum tank. By adding it inside, a high Ca yield can be obtained. During vacuum degassing
Since the boiling point of Ca changes in accordance with a change in pressure in Ca addition, a high Ca yield can be obtained by maintaining the inside of the vessel at a degree that does not hinder reflux, that is, at a vacuum of 100 to 200 Torr. In addition, in order to minimize the vacuum degassing time after the addition of Ca, if the method of adding immediately before the end of the vacuum degassing is adopted, since the molten steel flows even after the end of the vacuum degassing, Ca can be made uniform by a treatment of about 2 minutes.

In order to add a Ca alloy, it is necessary to overcome the upward flow of the molten steel generated by the vacuum degassing process and land the Ca alloy on the molten steel. Therefore, conventionally, the expensive but large specific gravity FeCa alloy, FeCaSi alloy, CuCa alloy, NiCa alloy, etc. have been used, but even inexpensive CaSi alloy
If grains are used, a high Ca yield can be obtained.

[0017]

Embodiment 1 The details of the method of the present invention will be described below with reference to FIG. 1 showing an embodiment. FIG. 1 is a schematic explanatory view of the RH method for implementing the method of the present invention. In FIG. 1, reference numeral 1 denotes a vacuum tank, and 2 denotes an immersion pipe of the vacuum tank 1, which includes an ascending pipe 2a and a descending pipe 2b. 3 is capacity
The 170-ton scale ladle 4 is a reflux gas injection tuyere provided inside the riser 2a.If argon gas is blown during vacuum degassing, the molten steel 5 in the riser 2a is raised based on the gas lift principle. , Removes hydrogen in the molten steel 5 and
Is configured to descend and recirculate. Reference numeral 6 denotes a water-cooled lance vertically inserted from the upper part of the vacuum chamber 1, which is provided with a nozzle having an inner diameter of 25 mm at the tip, and is configured to blow the desulfurizing agent powder together with the carrier gas onto the molten steel 5 in the vacuum chamber 1. 7 is an exhaust pipe.

With the above-described structure, the molten steel 5 in the ladle 3 at 1640 to 1680 ° C. where Al deoxidation has been completed is placed in a vacuum tank.
After the immersion tube 2 was immersed, the inside of the vacuum chamber 1 was evacuated from the exhaust tube 7 to a degree of vacuum of 2 Torr. Next, the circulating gas injection tuyere
Argon gas was blown in at 1000 Nl / min from 4 to raise the molten steel 5 in the riser 2a based on the principle of gas lift, remove hydrogen in the molten steel 5, and lowered the molten steel 5 from the downcomer 2b to recirculate. After the reflux of molten steel 5 was stabilized, CaO: 85%, CaF ₂ : 1 from the nozzle adjusted to 2.0 m distance between the molten steel 5 surface and the water cooling lance 6 tip
5% desulfurizing agent powder with a particle size of about 0.15 mm
Desulfurization treatment was performed by blowing upward at a feed rate of about 130 kg / min together with 5 Nl / min. The desulfurization rate in this case was 90%.

After the desulfurization treatment, the degree of vacuum when the CaSi alloy was added and the reflux time after the addition were variously changed, and the Ca yield was measured. The results are shown in Table 1. The CaSi alloy used was
A particle having a particle size of 7 to 20 mm was used at 2 kg / Ton.

[0020]

[Table 1]

As shown in Table 1, as the degree of vacuum decreases, C
aThe yield is improved, and the shorter the reflux time after the addition of the CaSi alloy, the higher the Ca yield. However, in order to homogenize the added Ca, a reflux of at least 1.5 min is required.

Example 2 The Ca yield and the Ca yield were measured with the Ca alloy addition and the Ca alloy addition particle size varied, and the Ca alloy addition with a vacuum of 160 Torr, a reflux time of 4 min, and a Ca alloy addition of 2 kg / Ton. The results are shown in Table 2.

[0023]

[Table 2]

As shown in Table 2, even when a CaSi alloy having a low specific gravity was used as the Ca alloy, the same Ca yield could be obtained by increasing the grain size as when using a FeCaSi alloy having a high specific gravity. .

[0025]

As described above, according to the method of the present invention, the desulfurizing agent powder is blown upward in the vacuum degassing process by the RH method.
And then desulfurization treatment, the pressure within the range that does not stop the reflux of the molten steel
By adding calcium alloy particles by force, highly clean steel can be efficiently produced without nitrogen pickup or a decrease in molten steel temperature.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of an RH method for carrying out the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum tank 2 Immersion pipe 2a Up pipe 2b Down pipe 3 Ladle 4 Reflux gas injection tuyere 5 Molten steel 6 Water cooling lance 7 Exhaust pipe

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21C 7/04 C21C 7/064 C21C 7/10

Claims (1)

(57) [Claims] When performing a vacuum degassing process on molten steel that has been deoxidized by the RH method, a desulfurizing process is performed by blowing a desulfurizing agent upward together with a carrier gas from a lance vertically inserted from an upper portion of a vacuum tank. A method for producing clean steel, characterized by increasing the pressure level to a level that does not stop the reflux of molten steel and adding calcium alloy particles into a vacuum chamber.