JPH0146563B2 - - Google Patents

Description

Detailed Description of the Invention The present invention provides a method for deoxidizing and removing nonmetallic inclusions from molten steel, which reduces the amount of oxygen and nonmetallic inclusions in steel that are harmful to the toughness, fatigue resistance, cold workability, etc. of steel. The present invention relates to a method and apparatus thereof.

Controlling the amount and form of nonmetallic inclusions is important in the production of high-quality steel, and for this purpose, it is necessary to reduce oxygen in molten steel, which is one of the causes of nonmetallic inclusions, and to reduce oxygen floating in molten steel. It is necessary to separate and remove the non-metallic inclusions that are present.

In order to efficiently perform this deoxidation and removal of inclusions, various out-of-furnace refining methods have recently been implemented or proposed.

These are methods in which only preliminary refining is performed in a melting furnace, and final refining is performed in a ladle outside the furnace, and the present invention is also a type of this outside-furnace refining.

Next, the principles and characteristics of various conventional out-of-furnace refining methods will be briefly described.

Vacuum degassing method This is the most widely applied method for melting special steel, and its principle is that undeoxidized or semi-deoxidized molten steel is stirred under a high vacuum to violently induce a CO reaction, resulting in dehydrogenation and deoxidation. It is to do acid.

The resulting steel has extremely low hydrogen and oxygen contents and has few nonmetallic inclusions, but the weak point is that the molten steel must be directly exposed to vacuum after removing the slag layer, so the nonmetallic inclusion particles are suspended. is not removed by adsorption to the slag, and problems related to non-metallic inclusions cannot necessarily be solved. Another drawback is that they typically require a vacuum of around 1 Torr, which requires a large capacity steam ejector, resulting in extremely high energy costs.

Ladle Furnace Method The purpose is to deoxidize, desulfurize, and remove inclusions.The principle is that the ladle has a structure similar to that of the Erou type arc furnace, and carbide slag refining is carried out.In order to accelerate the reaction, the ladle furnace is Gas bubbling is performed by injecting active gas to an extent that does not destabilize the arc.

This method yields steel of sufficient quality, but not only is the equipment cost quite high, but the reaction rate is slow, so the processing time is long, and as a result, the operating costs such as heating power, refractories, electrode rods, etc. are also quite high. There are drawbacks.

Gas bubbling method The main purpose is to equalize the temperature and remove inclusions, and the principle is that inert gas is blown into molten steel that has already been deoxidized through a breathable refractory, and the process is boiled to remove suspended non-metallic inclusions. This is to remove substances by adsorption to the slag.

However, although the work is simple and the cost is low, both deoxidation and removal of inclusions are insufficient. The reason is that the inert gas bubbles that are blown in do not induce a strong CO boiling reaction unlike vacuum degassing.
This is because the molten steel is oxidized by the atmospheric air.

Ca alloy injection method The principle of this method is to inject Ca alloy powder together with inert gas directly into molten steel through a refractory pipe, and at the same time cover the surface of the molten steel with non-oxidizing basic slag to remove oxidation, desulfurization, and inclusions. It is.

Although the quality level of the obtained steel and the reaction rate are favorable, and the equipment cost is not too high, there are drawbacks to the operating cost because a large amount of expensive Ca alloy and argon gas are required.

It can be seen that each of the above-mentioned conventional methods has advantages and disadvantages, and that the cost increases when high quality is sought.

On the other hand, the conditions for effectively deoxidizing steel and removing inclusions are summarized as follows.

(b) The molten steel to be processed requires appropriate preliminary refining depending on the refining method, processing time, and target level of refining.

(b) In order to increase the rate of deoxidation and removal of inclusions, stirring of molten steel is essential, and a strong CO boiling reaction such as vacuum degassing is desirable.

(c) Covering the molten steel with slag to adsorb and remove non-metallic inclusions. The slag composition is non-oxidizing, and to prevent rephosphorus and resulfurization,
Must be basic.

(d) During refining, oxidation of molten steel and slag must be completely prevented. The FeO concentration in the slag is preferably 1% or less.

The present invention was made in order to solve the problems of the conventional method described above, and provides conditions for effectively deoxidizing steel and removing non-metallic inclusions, thereby achieving a high degree of deoxidation and removal of non-metallic inclusions. It is an object of the present invention to provide a method for deoxidizing and removing non-metallic inclusions from molten steel, which can easily produce steel of a standard quality at low cost, and an appropriate deoxidizing and removing non-metallic inclusion device used therein. .

The inventors of the present invention have observed in detail the influence of atmospheric pressure on the boiling phenomenon in gas bubbling, and have discovered the following important facts by analyzing the reaction.

In other words, initial conditions of molten steel, slag composition, properties,
It has been found that effective deoxidation and removal of inclusions can be achieved by optimizing conditions such as bubbling strength and atmospheric pressure, and the present invention was constructed by elucidating these appropriate conditions.

The gist of the present invention is to induce a strong CO boiling reaction similar to vacuum degassing and to secure a non-oxidizing atmosphere by performing this at a low pressure close to vacuum based on the gas bubbling method. At this time, in order to effectively remove inclusions,
Boiling treatment of semi-deoxidized molten steel with appropriate slag. In order to significantly reduce operating costs, for example, a water ring type vacuum pump can be used to obtain the minimum necessary degree of vacuum.

The first aspect of the present invention is to place semi-deoxidized molten steel into a ladle, cover the surface of the molten steel with non-oxidizing and basic slag, and reduce the atmospheric pressure above the molten steel to 30 to 30°C.
This is a method for deoxidizing and removing non-metallic inclusions from molten steel, which is characterized by carrying out gas bubbling treatment at 150 Torr and blowing inert gas from the bottom of the ladle for 3 minutes or more.

A second invention of the present invention is an apparatus used in the first invention described above, in which the side wall has an airtight structure, vacuum covers are set on the top and bottom to make the whole airtight, and an inert gas is blown into the bottom part. A ladle equipped with a device, a water ring type vacuum pump connected to the upper vacuum cover via an exhaust duct, a filter type dust removal device attached to the upstream side of the vacuum pump, and a filter type dust removal device also attached to the downstream side of the vacuum pump. This is a device for deoxidizing and removing non-metallic inclusions from molten steel, characterized by comprising a water sealing control device that circulates water sealing water and maintains the water temperature at 30°C or less.

Hereinafter, the present invention will be explained by examples using the drawings.

FIG. 1 is a longitudinal sectional view showing an embodiment of the device of the present invention. In the figure, 1 is a ladle, and a porous plug 11 made of breathable refractory material is provided at the bottom for blowing inert gas. The steel side wall 2 of the ladle 1 has an airtight structure, and an upper airtight cover 3 is set at the upper part, and a lower airtight cover 4 is set at the lower part, making the entire ladle 1 airtight.

A filter type dust removal device 6 is connected to the upper airtight cover 3 via an exhaust duct 5. The dust removal device 6 removes dust by passing exhaust gas through an internal filter 7.

The dust-removed exhaust gas passes through an exhaust duct 8 and is exhausted by a water ring vacuum pump 9. The ultimate vacuum level of water ring pumps is not very good, but the vacuum level is 30~
Suitable for 150Torr, low operating cost and easy maintenance.

A water seal control device 10 is attached to the water seal vacuum pump 9 on the downstream side. The water sealing control device 10 circulates the water sealing water of the vacuum pump 9 and keeps the water temperature at 30°C.
It is to be kept below ℃.

Next, a method for refining molten steel according to the present invention using the refining apparatus according to the present invention configured as described above will be described.

First, in a melting furnace (e.g. arc furnace), molten steel is
The steel is pre-deoxidized with Mn and/or Si and tapped into a ladle 1 together with non-oxidizing and basic slag.

Place the ladle 1 on the lower airtight cover 4,
Next, the upper airtight cover 3 is set on the ladle 1. Operate water ring vacuum pump 9, and ladle 1
The air inside is exhausted, and the air is exhausted while removing dust with a dust removal device 6.

Reduce the pressure inside ladle 1 to a vacuum level of 30 to 150 Torr.
While holding _the ladle 1 at
Hold at 150 Torr for 3 minutes or more to perform strong gas bubbling (boiling) treatment.

In this case, when the atmospheric pressure drops below 200 Torr, the bubbling phenomenon completely changes from a simple disturbance to an intense boiling phenomenon, and fine bubbles are generated over the entire surface of the molten steel and slag, forming a carmella shape of several hundred mm.
Climb up. Process in this state for 3 minutes or more, and finely adjust the ingredients as necessary in between.

Oxygen and nonmetallic inclusions in the molten steel are effectively removed by this reduced pressure and gas bubbling by the inert gas. When the deoxidation and removal of inclusions are completed, the vacuum pump 9 and the blowing of inert gas are stopped, the upper airtight cover 3 is removed from the ladle 1, and the mold is used for casting.

The semi-deoxidized molten steel used in the present invention, which has been pre-deoxidized in a melting furnace, has an oxygen content of 100±30 p.p.
m. is preferable. The reason for using semi-deoxidized molten steel is that non-oxidizing slag can be quickly generated in the furnace and CO boiling reaction can be induced when the pressure is reduced in the ladle, whereas non-deoxidized molten steel requires a longer refining time in the ladle. This is because it is not appropriate.

Furthermore, in the present invention, the reason why the slag is made non-oxidizing is to prevent the molten steel from being oxidized by the slag during deoxidation and removal of non-metallic inclusions, and the reason why the slag is made basic is to prevent the molten steel from being oxidized by the slag. This is to prevent rephosphorus and resulfurization during deoxidation and removal of nonmetallic inclusions.

The reason why the atmosphere above the molten steel is set to 30 to 150 Torr is that if it exceeds 150 Torr, the deoxidation rate will be low and the processing time to reach a predetermined degree of deoxidation will be longer, which will be disadvantageous in terms of operating costs. On the other hand, the deoxidation rate is naturally higher and more advantageous at lower pressures, but at pressures below 30 Torr, water ring pumps cannot reach pressures lower than this, and other types of vacuum evacuation equipment, such as steam and ejectors, are required. In this case, the energy cost for operation becomes excessive, which deviates from the purpose of cost reduction.

In general, the lower the pressure, the higher the deoxidation rate, but when slag coexists, the effect becomes smaller, and the fact that the effect of a degree of vacuum below 30 Torr is not very large is also the basis for the above pressure limitation. .

Further, in the present invention, the inert gas includes, for example, N ₂ ,
Ar, hydrocarbon gas, etc. are used, and they do not cause harmful chemical reactions to molten steel and slag.
This is simply to obtain effects such as physical stirring and adsorption.

The reason why the gas bubbling treatment is carried out for 3 minutes or more is because the deoxidation rate is about 10 p.pm/min, and it is difficult to obtain a predetermined degree of deoxidation in less than 3 minutes.

Next, in the device of the present invention, the reason why the side wall of the ladle is made airtight is that if the space volume to be evacuated is designed to be as small as possible, a predetermined degree of vacuum can be reached in a short time, and as a result, deoxidation and removal of nonmetallic inclusions occur. This is because it is most appropriate to use the ladle itself as a vacuum tank, although the time can be shortened.

Also, the reason why a water ring type vacuum pump is used as an evacuation device is that compared to an ejector type, a mechanical vacuum pump has an overwhelming advantage in operating cost. When used for metal inclusions, maintenance is extremely disadvantageous due to the large amount of dust and heat.
Among them, the water ring type vacuum pump does not have a very good vacuum level, but it is the easiest to maintain.

In addition, the reason why a filter-type dust removal device was attached to the upstream side of the vacuum pump is that it is necessary for maintenance of the vacuum pump and to prevent contamination of the seal water with dust. This is because the filter type is practical, although it cannot be applied and there is some pressure loss.

Furthermore, the reason why a water sealing control device was installed downstream of the vacuum pump to circulate water sealing water and keep the water temperature below 30℃. This is because the ultimate vacuum level deteriorates, so it is necessary to constantly send sealed water at a low temperature (below 30°C) to the vacuum pump.

Next, examples of the present invention will be described.

Example In a melting furnace, molten high carbon steel is pre-deoxidized with Mn and Si under non-oxidizing and basic slag to an oxygen content of 100±30 p.pm, and then the molten steel is deoxidized with slag as shown in Figure 1. The steel was tapped into ladle 1 of the refining equipment as shown.

The ladle 1 was quickly made airtight, and the water ring vacuum pump 9 was operated to reduce the pressure of the atmosphere above the molten steel, and Ar gas was blown into the ladle 1 from the bottom.

The relationship between the time during this period, the change in atmospheric pressure, and the drop in molten steel temperature is as shown in FIG. From the figure, it can be seen that the pressure drops rapidly and becomes constant after about 2 minutes.

Furthermore, the relationship between the bubbling treatment time and the change in oxygen content of molten steel is as shown in FIG. The figure shows that deoxidation progresses rapidly.

The bubbling phenomenon becomes an intense boiling phenomenon when the atmospheric pressure is less than 200 Torr, and deoxidation and removal of nonmetallic inclusions proceed rapidly.

Next, the bubbling process according to the present invention described above is carried out from 8 to 8.
A wire rod was produced from the molten steel heated for 14 minutes, and the nonmetallic inclusion score of the obtained wire rod was evaluated by the Michelin method.

For comparison, a wire rod manufactured from molten steel by the conventional gas bubbling method under atmospheric pressure was similarly evaluated.

The distribution of non-metallic inclusion scores for wire rods obtained by both methods is shown in FIG.

From the figure, it can be seen that the nonmetallic inclusion scores of the wire rod according to the present invention are distributed in a significantly smaller area and have very little variation, and are significantly improved compared to the conventional method.

Furthermore, the cleanliness of the same wire was investigated according to JIS G 0555 (1956), and the distribution state is shown in FIG.

From the figure, the method according to the present invention has better cleanliness and less variation than the method according to the conventional method.
It can be seen that the steel material is cleaned.

As described above, in the method for deoxidizing and removing non-metallic inclusions of the present invention, semi-deoxidized molten steel is placed in a ladle and the surface of the molten steel is covered with non-oxidizing and basic slag. In the case of inclusions, it prevents the molten steel from oxidizing, rephosphorousing, and resulfurizing, and also makes it easy for the inclusions to be adsorbed and removed, and also reduces the atmospheric pressure above the molten steel.
The temperature is set at 30 to 150 Torr, and inert gas is blown in from the bottom of the ladle to perform gas bubbling treatment for at least 3 minutes, so gas bubbling under reduced pressure in semi-deoxidized molten steel induces an intense CO boiling reaction similar to vacuum degassing. This effectively promotes deoxidation and removal of non-metallic inclusions, making it easy to obtain steel with a high level of deoxidation and removal of inclusions in a short time. Since a vacuum pump with low operating cost, such as a sealed vacuum pump, can be used, there is an advantage that the refining cost can be reduced.

Furthermore, as mentioned above, the deoxidizing/removing nonmetallic inclusions device of the present invention is made airtight as a whole by the airtight side walls and upper and lower airtight covers, and uses a ladle equipped with an inert gas blowing device, so the exhaust volume is small. It is small and can reach the desired degree of vacuum in a short time, and because it uses a water ring vacuum pump like the one mentioned above, a degree of vacuum of 30 to 150 Torr can be easily obtained at low operating costs, and maintenance is easy. A filter-type dust removal device is installed on the upstream side of the vacuum pump, which prevents contamination of the seal water with dust and improves the maintenance of the vacuum pump.Also, the water seal water is circulated downstream and the temperature of the water is controlled. Since it is equipped with a water sealing control device that maintains the temperature below 30℃, the ultimate vacuum level will not deteriorate.
The present invention has the advantage of providing an appropriate deoxidizing/removing non-metallic inclusions device that can be used in the deoxidizing/removing non-metallic inclusions method of the present invention to easily obtain predetermined operating conditions, at low operating costs, and with easy maintenance. be.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of the device of the present invention. FIG. 2 is a diagram showing the relationship between time, changes in atmospheric pressure in the ladle, and drop in molten steel temperature in an embodiment of the method of the present invention. FIG. 3 is a diagram showing the relationship between the bubbling treatment time and the change in the oxygen content of molten steel in an example of the method of the present invention. Fourth
The figure shows the distribution of nonmetallic inclusion scores for steel wire rods manufactured by the present invention and the conventional method.
The figure also shows the distribution of cleanliness of steel wire rods. DESCRIPTION OF SYMBOLS 1... Ladle, 2... Steel side wall, 3... Upper airtight cover, 4... Lower airtight cover, 5, 8... Exhaust duct, 6... Filter type dust remover, 7... Filter, 9... Water ring type vacuum pump, 10... Water sealing control device, 11...Porous plug.

Claims (1)

[Claims] 1. Oxygen content of carbon steel, especially high carbon steel, is reduced to 100%.
Semi-deoxidized molten steel adjusted to ±50 ppm is placed in a ladle, the surface of the molten steel is sufficiently covered with non-oxidizing and basic slag, and the atmospheric pressure above the molten steel is set to 30~30 ppm.
150 Torr, inert gas is blown from the bottom of the ladle to induce an intense CO gas boiling reaction, and gas bubbling treatment is performed for 3 minutes or more and 10 minutes or less. Method. 2. The side wall is airtight, airtight covers are set on the top and bottom to make the whole airtight, the ladle is equipped with an inert gas blowing device at the bottom, and the water-sealed type is connected to the upper airtight cover via an exhaust duct. Equipped with a vacuum pump, a filter-type dust removal device attached to the upstream side of the vacuum pump, and a water sealing control device that circulates water sealing water and maintains the water temperature at 30 degrees Celsius or less, which is also attached to the downstream side. A device for deoxidizing and removing non-metallic inclusions from molten steel.