JPH0798972B2 - Ultra low carbon steel melting method - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is to decarburize undeoxidized or weakly deoxidized molten steel produced in a steelmaking furnace by using the RH method, the DH method, the VOD method, etc., and the carbon concentration in the steel is less than 10 ppm. The present invention relates to a method for producing ultra-low carbon steel by vacuum decarburization, which can rapidly produce the ultra-low carbon steel without impairing the operability of the apparatus.

[0002]

2. Description of the Related Art In recent years, continuous annealing equipment has been actively used from the viewpoint of shortening the annealing process of cold-rolled steel sheets and improving efficiency. Ultra-low carbon steel having a carbon concentration (weight fraction, hereinafter referred to as [C]) of 10 ppm to several ppm has been required as a material suitable for this.

Ultra-low carbon steel is conventionally decarburized from molten steel decarburized in a converter to [C] = 0.02 to 0.05% by weight under reduced pressure using a vacuum degassing apparatus such as the RH method. Has been melted by the method. In decarburization with a vacuum degasser, the decarburization rate gradually decreases in the extremely low carbon region of [C] <50 ppm. Therefore, it is necessary to industrially produce a large amount of extremely low carbon steel such as [C] <10 ppm. Was difficult. For example, in the case of decarburizing undeoxidized molten steel melted in a converter using a RH vacuum degassing apparatus to [C] <10 ppm, even if measures such as increasing the reflux rate are taken, it is 30-40 It required decarburization for a long time such as more than a minute.

Since the rate-determining process of the reaction in such an extremely low carbon concentration region is considered to be a mass transfer process of carbon in the molten steel to the reaction site, the reaction interface area is increased to increase the reaction rate. Attempts have been made to improve. The reaction sites in this case are assumed to be bubbles in the molten steel / melted steel interface, the surface of the steel bath in the degassing container, splashes that accompany bubbles when they leave the steel bath, but their contributions are not always clear. However, from the viewpoint that increasing the amount of stirring or refluxing Ar gas would be effective for the above three points,
The technology in which a large amount of Ar gas as high as 5 Nm ³ / min is blown into the molten steel is still used in the RH degasser.

Further, as a method for promoting the decarburization reaction in an extremely low carbon concentration range, a large amount of hydrogen is added to the molten steel during the vacuum decarburization treatment to actively generate bubbles in the degassing vessel. A method for increasing the gas-liquid interface area and promoting the decarburization reaction is disclosed in JP-A-57-194206. In the method of adding the hydrogen-containing substance while continuing the vacuum decarburization treatment, in order to effectively promote the decarburization, the RH of 235 tons scale
In this case, it was necessary to blow a large amount of hydrogen gas of 0.2 to 1 kg / min into the molten steel through a porous refractory plug installed at the bottom of the molten steel pot. When the present inventors conducted experiments on this method using a RH degassing apparatus on a scale of 250 tons, in order to effectively promote the decarburization reaction, in order to maintain a hydrogen concentration in steel of about 5 ppm, 2. It is necessary to add a large amount of hydrogen gas equivalent to 5 Nm ³ / min to the molten steel.

[0006]

In the RH vacuum degassing apparatus, even if the method of increasing the reflux rate to increase the decarburization rate is used, it is still necessary to decarburize for 30 to 40 minutes or more to decarburize to [C] <10 ppm. Such a long decarburization process is required, resulting in a large increase in melting cost and a decrease in productivity.

In addition, in the method of adding hydrogen to molten steel during vacuum decarburization, it was necessary to add a large amount of hydrogen to molten steel in order to effectively promote decarburization. However, injecting such a large amount of hydrogen gas into the molten steel in the ladle using a porous refractory plug or injection lance installed at the bottom of the molten steel ladle is a problem of equipment damage due to the molten steel scattering due to gas stirring. It is considered to be difficult to carry out on an industrial scale from the standpoint of durability of gas blown tuyere, dip tube refractory, etc. In the case of an RH degasser, a method of blowing hydrogen gas from a recirculating gas blowing tuyere installed in a dip tube is also conceivable. However, such a method has a low hydrogen gas dissolution efficiency and is not used during decarburization treatment. Since the hydrogen concentration in steel increased only up to about 3 ppm and the effective hydrogen concentration in steel could not be maintained, the decarburization promoting effect was small.

The present invention rapidly decarburizes in the extremely low carbon range of [C] <10 ppm, and stabilizes ultra-low carbon steel of [C] <10 ppm, which has been difficult to achieve stable mass production in the past. It is intended to provide technology that can be melted. At that time, it will be possible to provide a method capable of achieving the above-mentioned problems on an industrial scale without causing deterioration in operability due to the attachment of a metal as in the conventional method and problems such as durability of tuyere and refractory materials. It is what

[0009]

Means for Solving the Problems The technical means of the present invention is to use an RH method for producing undeoxidized or weakly deoxidized molten steel produced in a steelmaking furnace.
When performing vacuum decarburization treatment using the DH method, VOD method, etc., after performing vacuum decarburization treatment to a predetermined carbon concentration in steel,
It is characterized in that the decarburization rate is reduced or the decarburization is substantially stopped, a hydrogen addition treatment for dissolving hydrogen in molten steel is performed to adjust to a predetermined hydrogen concentration, and then the vacuum decarburization treatment is performed again. It is a method of melting ultra low carbon steel. In this case, the hydrogenation treatment was performed at a time when [C] was 25 ppm or less, and the hydrogen concentration in the molten steel after the hydrogenation treatment (weight fraction, hereinafter referred to as [H]) was in a range satisfying the following equation (1). It is advisable to perform hydrogenation treatment so that

[H] ≧ ([C] − [C] final) / 5 + 4 (1) However, [H]: Hydrogen concentration in molten steel (ppm) [C] final: Target decarburization [C] value (ppm) ) Is.

During the hydrogenation process, the pressure in the degassing vessel should be set to 20 Torr or higher, and then the pressure in the degassing vessel should be set to 2 Torr or lower to perform the vacuum decarburization processing. Moreover, if the pressure in the degassing container is set to 30 Torr or more,
It is not always necessary to add the hydrogen-containing substance from a deep portion in the molten steel, and hydrogenation treatment may be performed by means of adding the hydrogen-containing substance on the molten steel bath surface in the degassing vessel.

Further, when the RH vacuum degassing device is used as the vacuum degassing device, as means for dissolving hydrogen in the molten steel, a tuyere for recirculating gas on the inner surface of the RH riser pipe, RH
Injection lance immersed in molten steel in the pan so that the blown gas floats in the rising pipe, gas blowing tuyere installed on the side of the RH degassing vessel, installed on the molten steel bath surface in the RH degassing vessel It is preferable to blow the hydrogen-containing substance from any one or more of the upper blowing lances of the water cooling structure which can be moved up and down.

When a vacuum decarburization process is carried out again after the hydrogenation process, if a RH degassing device is used, a recycle gas tuyere on the inner surface of the RH riser pipe and a gas blowing vane installed in a molten steel ladle. The decarburization can be further promoted by injecting the hydrogen-containing substance from at least one of the mouth and the injection lance immersed in the molten steel in the pot. As the hydrogen-containing substance, the hydrogenation treatment is performed using a substance containing at least one of hydrogen gas, water vapor, calcium hydroxide, aluminum hydroxide, and magnesium hydroxide.

[0014]

[Operation] As a method of promoting the decarburization reaction in the extremely low carbon concentration range, a large amount of hydrogen is added to the molten steel during the vacuum decarburization treatment to actively generate bubbles in the degassing vessel, and thereby the gas-liquid interface Regarding the method of increasing the area and accelerating the decarburization reaction, experiments were conducted mainly with a 250-ton scale RH degasser, and as mentioned above, there was a problem with the method of adding hydrogen, and this method should be applied on an industrial scale. Was difficult. Therefore, as a result of the inventors' research on an effective hydrogenation method for promoting decarburization, the present invention has been invented.

In the method of adding the hydrogen-containing substance while continuing the vacuum decarburization treatment, the dehydrogenation reaction occurs at the same time as the decarburization reaction. Therefore, it is fast to maintain a high [H] effective for promoting decarburization. It was necessary to add hydrogen at. However, the inventors of the present invention can obtain a sufficient effect to promote decarburization if a predetermined [H] or more [H] is obtained in a predetermined [C] range, and it is necessary to maintain a high [H] for a long time. Found that there is no. Keeping [H] at a high value during the vacuum degassing process causes a difficulty in operation in the existing equipment due to the problem of the hydrogen addition rate as described above, but temporarily suspends the vacuum degassing process. If only H] is increased, it can be easily realized at a relatively low addition rate.

Further, after adjusting to appropriate [C] and [H] by a hydrogenation treatment in which hydrogen is added while suppressing the degassing reaction by lowering the operating vacuum degree at an appropriate time during the decarburization treatment, It is also possible to effectively promote decarburization by a method of activating the degassing reaction by improving the operation vacuum degree again. In this case, [H] rises once in the hydrogenation process,
After improving the degree of vacuum, it drops rapidly and takes about 5 minutes.2.
It decreases to about 5 ppm. On the other hand, [C] does not change much during the hydrogenation treatment and rapidly decreases in the initial 5 minutes or so of the subsequent decarburization treatment, but with the decrease of [H], the decarburization promoting effect also gradually decreases. ] To about 2.5 ppm or less, the decarburization rate drops to the level of the conventional method.

Therefore, [H] <2.
[C] initial, [H] i at the start of re-decarburization that can decarburize to [C] <10 ppm until it reaches 5 ppm
The range of the initial is shown in FIG. If the conditions of higher hydrogen concentration and lower carbon concentration than the curve of FIG. 2 are selected, [C] <10 pp
Although it is possible to decarburize up to m, the conditions may be determined so that the total treatment time is the shortest in consideration of the decarburization rate and the hydrogen addition rate.

From FIG. 7, when hydrogen is added at [C]> 25 ppm, the effect of hydrogen addition gradually decreases, so it is necessary to further increase the hydrogen concentration.
It is not a good idea considering the efficiency of hydrogenation and the time required for hydrogenation. Moreover, in a normal vacuum degassing device, [C] =
Since the decarburization rate suddenly decreases from around 25 ppm, it is advantageous to apply the method of the present invention in the range of [C] <25 ppm.

FIG. 7 shows the decarburization target value [C] final of 10
It is a graph in the case of ppm, but in this range, [H] init before re-decarburization treatment is within the range that satisfies the above formula (1).
It can be seen that if ial is adjusted, decarburization can be promptly carried out below the decarburization target value. In order to shorten the overall processing time, it is necessary to add hydrogen at a high speed to shorten the time for hydrogenation processing. In the method of the present invention, the dehydrogenation reaction is suppressed by keeping the pressure in the degassing vessel during the hydrogenation treatment at a high value, and the high speed addition is possible by absorbing the hydrogen gas in the molten steel in the degassing vessel. [H] =
When adding up to 7ppm, equilibrium hydrogen partial pressure is about 50To
Since it is rr, it is considered that the pressure inside the degassing vessel should be set to about 50 Torr, but if a suitable hydrogenation method is used, even at a pressure of 20 Torr, there is no substantial difference in the hydrogenation rate.

FIG. 8 shows a 250 t scale RH degassing apparatus in which 6.0 N of H ₂ gas is supplied from the tuyere of the circulating gas of the rising pipe.
FIG. 3 is a diagram showing the relationship between the pressure in the degassing vessel and the dissolution efficiency of hydrogen gas when bubbling m ³ / min and Ar gas at 1.0 Nm ³ / min. Here, [H] is in the range of 3 to 7 ppm. In the conventional method, it was possible to blow a large amount of hydrogen gas into the dipping tube itself, but a sufficiently effective hydrogen concentration could not be obtained because the hydrogen gas dissolution efficiency was low. On the other hand, according to the method of the present invention, the dissolution efficiency is high even when a large amount of hydrogen gas is blown from the immersion pipe.

Further, the pressure inside the degassing vessel is set to 30 Torr.
According to the above, even if the hydrogen-containing substance is added from a relatively shallow portion in the molten steel in the degassing vessel or on the surface of the molten steel bath in the degassing vessel, it is possible to add hydrogen efficiently. According to this method, it is possible to inject hydrogen gas at a higher speed without being relatively restricted by facilities or operations, for example, by using an upper blowing lance.

FIG. 9 shows a 250t scale RH degassing apparatus, in which molten steel having an initial [H] concentration of about 2 ppm was removed from a height of 2.0 m from the molten steel bath surface by using an upper blowing lance charged in the degassing vessel. It is the figure which showed the relationship between [H] density | concentration and degassing container internal pressure after 5 minutes when hydrogen gas was top-blown at the blowing speed of 10 Nm < ³ > / min. In the same figure, the relationship between the hydrogen partial pressure and the equilibrium value [H] at 1600 ° C is also shown.

When the RH degasser is used, the means for adding hydrogen during the hydrogenation process is as shown in FIGS.
It is important to supply a high-speed hydrogen-containing substance into the degassing vessel or through the molten steel into the degassing vessel by an operationally easy method. That is, (a) Tuyere 4 for recirculating gas on the inner surface of the rising pipe (Fig. 1) (b) Injection lance 5 (Fig. 2) immersed in molten steel in the pan so that the blown gas floats in the rising pipe (Fig. 2). Upward-blowing lance 6 of water-cooling structure installed on the molten steel bath surface in the degassing vessel 1 (Fig. 3) (d) Gas blowing tuyere 7 installed on the side of the degassing vessel
(Fig. 4) and the like enable high-speed gas injection as described above. Further, by using a plurality of means among these means, it is possible to dissolve hydrogen at a higher speed.

Continuing the addition of hydrogen during the re-decarburization treatment subsequent to the hydrogenation treatment is effective for maintaining the high [H] state for a little longer time and further promoting the decarburization.
As the hydrogen addition means at this time, since the dissolution efficiency of hydrogen in the degassing vessel is significantly reduced, the dissolution rate is improved by blowing a hydrogen-containing substance into the molten steel to make the levitation distance as long as possible, Moreover, it is essential that the method is easy in operation. In the case of the RH degasser, the tuyere for circulating gas on the inner surface of the riser pipe (Fig. 1), the gas blowing tuyere installed in the ladle (Fig. 5), the injection lance immersed in the molten steel in the ladle ( By adding a hydrogen-containing substance at an appropriate rate using, for example, FIG. 6), it is possible to dissolve hydrogen to some extent effective without hindering the operation. Further, by using a plurality of means among these adding means, it is possible to accelerate the decarburization by increasing the dissolution rate of hydrogen while mitigating the problem at the time of high speed addition of hydrogen.

As the above-mentioned hydrogen-containing substance, not only gas containing hydrogen gas but also water, steam, calcium hydroxide and the like are immediately dissociated and dissolved in the molten steel as hydrogen, so that the same effect can be obtained.

[0026]

EXAMPLE An example in which the method of the present invention is carried out in an RH degasser having a scale of 250 tons will be shown. 250 tons of undeoxidized molten steel having about 350 ppm of [C] and about 450 ppm of oxygen concentration melted in a converter was decarburized by using an RH degasser.

Table 1 shows examples and comparative examples. Example
1 is the tuyere 4 for exhaust and recirculation gas in the degassing container 1
Ar gas 2.0 Nm³ / Min.
The charcoal treatment was performed for 12 minutes. After that, there are 6 stages of exhaust ejector
The operation of the Kuta is partially stopped and the pressure inside the degassing container is set to about 30T.
As the orr, the decarburization rate was reduced to
H from the tuyere 4 for the circulating gas of the rising pipe₂ Gas is 6.0 Nm³ /
Min, Ar gas 1.0 Nm³ / Min, blow for 3 minutes, hydrogen
An addition process was performed. [H] is about 1 by hydrogenation
It increased from ppm to about 7 ppm. Exhaust stopped after that
While restarting the ejector, H ₂ Stop gas blow
Stop and Ar gas only 2.0Nm³ Per minute
It was blown from the tuyere 4 for soot and re-decarburized for 5 minutes. Again
Before starting decarburization, [C] is about 25ppm on average.
there were. After restarting the exhaust ejector, the internal pressure of the degassing container
The force dropped below 2 Torr within 1 minute. Recarburization
After treatment, [C] averages about 8 ppm and [H] averages about 8 ppm.
It was 3 ppm. After the decarburization treatment, Al deoxidation treatment is performed 5
Continued for a minute.

In Example 2, as in Example 1, the usual decarburization treatment was performed for 12 minutes, followed by hydrogenation treatment for 3 minutes, re-decarburization treatment for 5 minutes, and Al deoxidation treatment for 5 minutes. . Unlike in Example 1, R as shown in FIG.
1.0 Nm ³ / min of H ₂ gas from the injection lance 5 immersed below the H ascending pipe, 2.5 Nm ³ / min of H ₂ gas from the tuyere 4 for the circulating gas of the ascending pipe, and Ar gas of 1.
5 Nm ³ / min each was blown in, and hydrogen addition was continued.
Before re-decarburization treatment, [C] had an average of about 25 ppm and [H] had an average of about 7 ppm. After completion of the re-decarburization treatment, [C] had an average of about 6 ppm and [H] had an average of about 4.5 ppm.

In Example 3, the same decarburization treatment as in Example 1 was performed for 12 minutes, and then the pressure in the degassing vessel was adjusted to about 30 To.
As rr, hydrogenation treatment was performed for 3 minutes, and then the ejectors in all stages were operated again to perform re-decarburization treatment for 5 minutes and Al deoxidation treatment for 5 minutes. As in the hydrogenation process are shown in FIG. 3, the H ₂ gas from the recirculation gas tuyere 4 in the riser 2.5
Nm ³ / min, 2 Ar gas 1.5 Nm ³ / min respectively with blown, the lance over a water-cooled structure having one vertically downward Laval nozzle from the virtual bath surface of the degassing vessel.
It was lowered to a height of 5 m and H ₂ 10 Nm ³ / min was sprayed onto the molten steel bath surface. During the recarburization process, the upper blowing lance is raised,
H ₂ gas from circulating tuyere 4 _2.5 Nm ³ / min and A
Blow of 1.5 Nm ³ / min of r gas was continued. Before re-decarburization, [C] is about 25ppm on average, and [H] is about 7p on average.
It was pm. [C] after re-decarburization is about 7pp on average
m and [H] were on average about 3.8 ppm.

Comparative Example 1 is a case where the same RH degassing apparatus as in Example 1 was used to carry out a normal decarburizing treatment for 20 minutes and an Al deoxidizing treatment for 5 minutes without hydrogenation. [C] after the decarburization treatment was about 17 ppm on average. In Comparative Example 2, using the same RH degassing apparatus as in Example 1, after the normal decarburizing treatment for 5 minutes, the decarburizing treatment was performed for 15 minutes while adding hydrogen, and the Al deoxidizing treatment was performed for 5 minutes. That is the case. Hydrogen addition during decarburization is done by the tuyere 4
To H ₂ gas at 6.0 Nm ³ / min and Ar gas at 1.0 N
Each m ³ / min was blown in, and during that time, the exhaust ejectors of all stages were operated in the same manner as in the ordinary decarburization treatment. After decarburization, [C] averages about 12 ppm and [H] averages about 3.
It was 5 ppm.

Table 1 shows the average values and standard deviations of the [C] values after the decarburization treatment. In any of the examples, [C] <
Decarburization up to 10 ppm is possible with little variation.

[0032]

[Table 1]

[0033]

Industrial Applicability According to the present invention, decarburization in an extremely low carbon area can be carried out rapidly, and as a result, it becomes possible to stably produce a large amount of [C] <10 ppm of extremely low carbon steel. Further, in the method of the present invention, there is no risk of equipment damage due to splashing of molten steel and operational obstruction factors such as abnormal wear of refractories, and further, it can be carried out with a small modification by simply supplying hydrogen gas to the gas blowing pipe of the existing equipment. Therefore, it is widely industrially applicable.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 2 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 3 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 4 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 5 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 6 is a schematic cross-sectional view of an RH device showing an aspect of equipment for carrying out the present invention.

FIG. 7 [C] after hydrogenation treatment and before re-decarburization treatment
It is a graph which shows the suitable range for obtaining [C] <10 ppm of [H].

FIG. 8 is a graph showing the relationship between the pressure in the degassing container and the hydrogen gas dissolution efficiency when hydrogen gas is blown from the recirculating gas tuyere of the RH riser pipe.

FIG. 9 is a graph showing the relationship between the pressure inside the degassing container and the [H] concentration when hydrogen gas is blown upward in the degassing container.

[Explanation of symbols]

1 Degassing Container 2 Molten Steel Pan 3 Molten Steel 4 Tufts for Circulating Gas 5 Injection Lance 6 Top Blowing Lance 7 Gas Blowing Tuyer 8 Porous Plug

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaru Washio 1 Kawasaki-cho, Chiba City Kawasaki Steel Co., Ltd. Chiba Steel Works (72) Inventor Kazuhisa Hamaue 1 Kawasaki-machi Chiba City Kawasaki Steel Co., Ltd. Chiba Steel Works (72) ) Inventor Hiroshi Nishikawa 1 Kawasaki-cho, Chiba City Kawasaki Steel Works Chiba Works

Claims (7)

[Claims] 1. When decarburizing molten steel with a vacuum degassing apparatus to produce ultra-low carbon steel, the decarburizing rate is reduced or substantially reduced after vacuum decarburizing treatment to a predetermined carbon concentration in steel. A method for melting ultra-low carbon steel, characterized in that the decarburization is stopped temporarily and hydrogen is added to the molten steel to adjust the hydrogen concentration in the specified steel, and then vacuum decarburization is performed again. . 2. The hydrogen concentration in molten steel [H] (ppm) after the hydrogenation treatment is performed when the average carbon concentration [C] (ppm) in the molten steel is 25 ppm or less.
The method for smelting ultra-low carbon steel according to claim 1, wherein the hydrogenation treatment is performed so as to satisfy the following formula. [H] ≧ ([C] − [C] final) / 5 + 4 where [C] final: decarburization target [C] value (pp
m) 3. After performing vacuum decarburization treatment to a predetermined carbon concentration in steel, the hydrogenation treatment is performed while the pressure in the degassing vessel is set to 20 Torr or higher, and then the pressure in the degassing vessel is set to 2 Torr or lower and vacuumed. The method for melting ultra low carbon steel according to claim 1 or 2, wherein decarburization treatment is performed. 4. The hydrogenation treatment is a means for adding a hydrogen-containing substance onto the molten steel bath surface in the degassing vessel while maintaining the pressure in the degassing vessel at 30 Torr or higher. The melting method of the ultra-low carbon steel according to any one of 1 to 3. 5. An RH vacuum degassing device is used as the vacuum degassing device, and as the hydrogenation treatment, a recycle gas tuyere on the inner surface of the RH rising pipe and a molten steel in a pan so that the blown gas floats in the RH rising pipe. Injection lance immersed inside, RH degassing vessel tuyere installed on the wall surface, RH
5. The hydrogen-containing substance is blown from at least one blowing means of an upward blowing lance of a water cooling structure installed on the molten steel bath surface in the degassing vessel. Ultra low carbon steel melting method described. 6. An RH degassing device is used as the vacuum degassing device, and after the hydrogenation treatment, a tuyere for recirculating gas on the inner surface of the RH riser pipe, a gas blowing tuyere installed in a molten steel ladle, and a molten steel in the ladle. The method for melting ultra-low carbon steel according to any one of claims 1 to 5, wherein the hydrogen-containing substance is blown from at least one blowing means of the injection lance immersed in the. 7. The hydrogen-containing substance is hydrogen gas,
Water, steam, calcium hydroxide, aluminum hydroxide,
The hydrogenation treatment is performed by using one containing at least one of magnesium hydroxide.
7. The melting method of the ultra-low carbon steel according to any one of to 6.