JPH07224313A - Production of high clenliness steel utilizing co deoxidation - Google Patents

Abstract

PURPOSE:To produce an extra-low oxygen steel by reducing free [O] to an extra-low level by CO deoxidation and suppressing the amt. of Al to be added as a deoxidizer. CONSTITUTION:At the tim of subjecting molten steel refined into [C] >=0.10wt.% to evacuating treatment by a vacuum degassing device while an oxygen or oxygen-contg. gas is fed to the molten steel held to an evacuating atmosphere of <=1330Pa by a flow rate of 0.03 to 0.30Nm<3>/min.ton, it is subjected to evacuating treatment into free [O]<=3Oppm. When [C] attains to 0.08 to 0.02, the feeding of oxygen is stopped, and next, Al is added thereto. Thus, Al is added to the molten steel having low free [O], by which the yield of Al to be added improves and the amt. of deoxidized product Al2O3 remaining in the steel is made small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high cleanliness steel by a reduced pressure treatment using CO deoxidation.

[0002]

2. Description of the Related Art Molten steel smelted in converters, electric furnaces, etc.
After the cleanliness is increased by depressurization using a vacuum degassing device such as H or DH, continuous casting is performed. As the degassing treatment, for example, in Japanese Patent Laid-Open No. 54-93618, low-vacuum treatment is performed to weakly deoxidize [C], and then Al is added to convert [C] from the converter steel into 0.03 to 0.03. 0.1
0% low carbon steel is melted. In RH vacuum degassing,
For example, a vacuum degassing apparatus whose equipment configuration is shown in FIG. 1 is used. The vacuum container 10 includes a pair of ascending pipe 11 and descending pipe 1.
2 is provided at the bottom of the container, and the inside 13 of the container is connected to an appropriate vacuum source. The rising pipe 11 is provided with a nozzle 14 for blowing an inert gas.

Molten steel 30 melted in a converter or an electric furnace is
The ladle 20 is charged. When the lower portions of the ascending pipe 11 and the descending pipe 12 are immersed in the molten steel 30 and the vacuum container 10 is evacuated, the molten steel 30 rises in the ascending pipe 11 and the descending pipe 12 according to the degree of vacuum inside the container 13. When the inert gas is blown from the nozzle 14, the apparent specific gravity of the molten steel in the rising pipe 11 becomes small. Therefore, the molten steel 30 is the upflow 31
As a result, the ascending tube 11 is raised and is sent to the vacuum container 10. The gas component contained in the molten steel 32 in the vacuum container 10 is released from the molten steel 32 to the atmosphere because the molten steel 32 is exposed to the vacuum atmosphere inside the container 13. The molten steel 32 having a reduced gas component has a large specific gravity and is returned as a downward flow 33 from the vacuum container 10 to the ladle 20. The molten steel 30 is repeatedly refluxed in the order of the ladle 20, the ascending pipe 11, the inside 13 of the container, the descending pipe 12, and the ladle 20, [H], [O].
Gas components such as H ₂ and CO are removed.

[0004]

The conventional depressurization treatment is [C] = 0.08 in an atmosphere exceeding a vacuum degree of 1330 Pa.
~ 0.04 molten steel is processed. The molten steel subjected to the depressurization treatment under these conditions has variations in free [O] in the range of 300 ± 100 ppm. Al is added to lower the [O] of the molten steel. Al is [Al] + [O] → A
The molten steel is deoxidized according to the reaction of l ₂ O ₃ , but the total [O] immediately after deoxidation is usually about 60 ppm or more. The deoxidation product Al ₂ O ₃ thus suspended in the molten steel is
It causes the long nozzle for continuous casting and the dipping nozzle to be blocked, resulting in continuous deterioration of castability. The obtained steel material also has poor cleanliness, and defects easily occur during rolling and processing. Therefore, for example, in the RH treatment, Al ₂ O ₃ is floated and separated by continuously refluxing the molten steel. However, in order to reduce the total [O] to 40 ppm or less, a reflux time of at least 5 minutes or more is required, which hinders productivity. Further, since there is variation in the total [O] of the molten steel before Al deoxidation, it is necessary to add a relatively large amount of Al, and the accuracy of [Al] after deoxidation deteriorates. The present invention has been devised to solve such a problem, and reduces the amount of Al added by deoxidizing molten steel with [O] reduced by decompression treatment using CO deoxidation. At the same time, it is an object of the present invention to suppress the deoxidation product Al ₂ O ₃ from remaining as inclusions as much as possible, and to manufacture low-carbon steel excellent in cleanliness at low cost and with high productivity.

[0005]

In order to achieve the object, the manufacturing method of the present invention is characterized in that when the molten steel refined to [C] ≧ 0.10 wt% or more is depressurized by a vacuum degassing apparatus,
0.03 for molten steel held in a reduced pressure atmosphere of 30 Pa or less
While supplying oxygen or an oxygen-containing gas at a flow rate of up to 0.30 Nm ³ / min · ton, the pressure is reduced to free [O] ≦ 30 ppm, and when [C] reaches 0.08 to 0.02 wt%, it is sent. Characterized by stopping the acid and then adding Al. In order to actively perform CO deoxidation during the pressure reduction treatment, it is preferable to use molten steel having a [C] before the pressure reduction treatment of 0.15 wt% or more. Molten steel [C] is adjusted by blow-stopping in a converter, an electric furnace, or the like. Further, in order to obtain the target product [C], [C] during depressurization is 0.08-
The acid transfer is stopped at a predetermined [C] concentration in the range of 0.02% by weight. In the present invention, for example, as shown in FIG. 2, a vacuum container 10 having an oxygen lance 40 penetrating an upper lid is used, and oxygen gas 41 or oxygen such as Ar—O ₂ is directed from the oxygen lance 40 toward the surface of the molten steel 32. A mixed gas (hereinafter represented by oxygen) is sprayed. The oxygen gas 41 is
According to the reaction of [C] + [O] → CO, it reacts with carbon in the molten steel, and CO gas which is a reaction product is released into the inside 13 of the container. Although oxygen is blown upward in FIG. 2, it is also possible to send oxygen by lateral blowing, oblique blowing, or the like through the dip tuyere, or by combining lateral blowing, oblique blowing, or the like with the upward blowing.

[0006]

The reduced pressure treatment according to the present invention uses molten steel refined to a medium carbon level of [C] before treatment of 0.10% by weight or more, preferably 0.15% by weight or more.
1330 Pa for molten steel with a high [C] of 0.10% by weight or more
When oxygen is supplied under the following reduced pressure atmosphere, [C] +
The decarburization reaction is promoted according to the reaction of [O] → CO. The CO gas generated by the decarburization reaction bubbles the molten steel when it floats in the molten steel, so that other gas components such as [H] and [N] are also removed. The effect of CO deoxidation is unexpectedly large, and free [O] decreases in a short time. As a result, molten steel having an extremely low [O] can be rapidly obtained. For example, when the oxygen transfer is stopped at the target carbon concentration of [C] = 0.08 to 0.02% by weight, the free [O] becomes 30 ppm or less. By adding Al to the CO-deoxidized molten steel, a total [O] ≦ 20 ppm of molten steel can be obtained. Since Al is added to molten steel whose free [O] is as low as 30 ppm or less, the Al yield is stable at a high level, and [O] ≦ 20 p
High cleanliness steel of pm or less is obtained. Moreover, since there are few deoxidation products such as Al ₂ O ₃ suspended in the molten steel, slabs can be manufactured with a high continuous casting ratio without causing blockage in long nozzles, immersion nozzles and the like. Moreover, the accuracy of the Al content is also improved.

Since the molten steel treated according to the present invention has a high [C] level, it is possible to set a large amount of acid transfer and maintain a large amount of decarburization per unit time. Further, the decarburization reaction is actively carried out, and a large amount of CO gas bubbles are generated in the molten steel. The CO gas bubbles activate stirring of the molten steel surface, generation of splash, and the like, and increase the gas-liquid reaction interface area. Thereby, denitrification of molten steel also progresses. That is, [C] of the molten steel before depressurization treatment is set to 0.10% by weight or more, and 0.0
Deoxidation and denitrification are promoted by feeding acid at a flow rate of ^{3 to} 0.30 Nm ³ / min · ton. The reduction of free [O] and the stirring action of molten steel by CO gas bubbles become remarkable when [C] at the time of tapping is 0.10 wt% or more.
When [C] is less than 0.10% by weight, the free [O] contained in the molten steel becomes high, and the harmful effect of oxygen adsorbed on the surface of the molten steel begins to appear.

In order to increase the gas-liquid interface area effective for the degassing reaction, it is necessary to maintain the oxygen supply rate within the range of 0.03 to 0.30 Nm ³ / min · ton. When the oxygen supply rate is less than 0.03 Nm ³ / min · ton, CO gas bubbles required for the disturbance of the interface and the increase of the reaction interface area are less generated, and the deoxidation / decarburization rate is reduced. As a result, a decompression process for a long time is required. Conversely, 0.30 Nm ³ / min
When the oxygen supply rate exceeds ton, the splash is intensely generated, and the amount of metal adhered to the inner wall of the vacuum container 10 and the oxygen lance 40 increases. In addition, the oxygen taken into the molten steel 32 increases, and the free [O] during processing is not kept low,
The cleanliness of molten steel decreases. The effect of acid transfer is vacuum 133
It becomes remarkable when the molten steel is exposed to a reduced pressure atmosphere of 0 Pa or less.
This is because when the degree of vacuum becomes 1330 Pa or less, the partial pressure of CO gas in the atmosphere decreases, and oxygen adsorbed on the surface of the molten steel and CO gas generated by the decarburization reaction are easily released into the atmosphere, resulting in deoxidation. -This is because the decarburization reaction is promoted.

When [C] is up to about 0.02% by weight, there is substantially no adverse effect due to free [O], so it is preferable to carry out degassing while continuing the acid transfer. But,
When the acid transfer is continued in the latter stage of the treatment in which the content of [C] is less than 0.02% by weight, the amount of free [O] incorporated in the molten steel increases and the adsorbed oxygen tends to have an adverse effect. Therefore,
[C] Discontinue the acid transport in the latter stage of the treatment in the carbon region of 0.02% by weight. Since denitrification and decarburization as well as denitrification have progressed sufficiently before the transfer of oxygen is stopped, the actual damage caused by the decrease in denitrification rate in the latter stage of the treatment that results in low carbon and high free [O] is avoided, and ultra-low nitrogen steel is used. It is melted with high productivity. Free [O] of the depressurized molten steel is sufficiently reduced to 20 ppm or less, so that the addition amount of Al necessary for deoxidation can be small. As a result, the addition of Al reduces the cleanliness A
Generation of l ₂ O ₃ is suppressed and total [O] is 30 pp
m or less. Also, the Al yield is stable at a high level of 70 to 80%. In this respect, the conventional depressurized molten steel is
[O] has a variation of 300 ± 100 ppm,
Immediately after deoxidation, the total [O] becomes 60 ppm or more. Moreover, the Al yield is as low as 40 to 60%.

[0010]

EXAMPLE An example in which the present invention is applied to denitrifying molten steel by using the RH degassing apparatus shown in FIG. 2 will be described.
However, the present invention is not limited to this, and the DH
Of course, the same applies to other types of degassing devices such as. Ladle 2 of free [O] = 76 ppm molten steel 30 stopped by blowing [C] = 0.26 wt% in a converter
Charged to 0. Next, the rising pipe 11 of the vacuum container 10
The downcomer pipe 12 was immersed in the molten steel 30, and the inside 13 of the container was evacuated to a maximum ultimate vacuum of 133 Pa. In this state, oxygen 41 was blown from the oxygen lance 40 onto the surface of the molten steel 32 for 25 minutes at a flow rate of 0.07 Nm ³ / min · ton.
After that, the supply of oxygen was stopped and a pressure reduction treatment was performed at 133 Pa for 2 minutes. The variation of [C] and free [O] was investigated over the whole period from the start to the end of the pressure reduction treatment.
The survey results are shown in Table 1.

[0011]

[Table 1]

The transitions of [C] and free [O] at this time are shown in FIG. 3 in relation to the CO partial pressure in the atmosphere. In FIG. 3, [C] = 0.068 wt% and [O] = 330 pp
A case where the molten steel blown to m is exposed to a vacuum atmosphere is also shown as a comparative example. As is clear from FIG.
In the comparative example in which the molten steel blown to the low carbon region of [C] = 0.068 wt% was subjected to the depressurization treatment, the free [O] after the treatment was at a high level exceeding 200 ppm, and the substantial C
O deoxidation could not be expected. On the other hand, [C] = 0.
In the example of the present invention in which the molten steel blown in the medium carbon region of 26% by weight was subjected to a reduced pressure treatment, deoxidation was possible up to free [O] = 20 ppm. However, when [C] became less than 0.02% by weight, there was a tendency that free [O] increased. From this comparison, when [C] = 0.1% by weight or more of molten steel is subjected to a reduced pressure treatment and when oxygen transfer is stopped in the carbon region of 0.02% by weight or more, extremely low [O] molten steel is obtained. I understand.

As shown in Table 1, the free [O] was 20 ppm after 11 minutes of the pressure reduction treatment. From these measured values, the amount of Al added necessary to fix free [O] is calculated to be 0.0022% by weight on the basis of molten steel. Therefore, Al was added to the molten steel by changing the added amount of the calculated value of 0.002% by weight or more. Before and after the addition of Al, auxiliary raw materials for component adjustment were added, and after the addition of Al, the molten steel was depressurized at 133 Pa for 2 minutes to complete the refining process. The refined molten steel was continuously cast, and the obtained slab was analyzed for its composition and its cleanliness was investigated. The cleanliness is J
Investigation based on IS G0555, d ₆₀ × 400
Expressed in (%). In addition, Al ₂ O ₃ is a
Only l ₂ O ₃ was extracted and separated and measured.

[0014]

[Table 2]

As is clear from Table 2 showing the survey results,
Addition of a small amount of Al results in a low proportion of Al ₂ O ₃ inclusions that reduce cleanliness, and [O] has an extremely low oxygen level of 10 ppm or less. Since the molten steel after deoxidation is a high cleanliness steel with few inclusions, the casting ratio was continuously improved to 1.5 times that of the conventional one. The yield of addition of Al was also stable at a high level, and a steel material having an Al content with high accuracy was obtained. In the above example, the case where the molten steel subjected to the converter refining is subjected to the depressurization treatment has been described. However, even when the molten steel melted in an electric furnace is depressurized, as long as the [C] of the molten steel is 0.10% by weight or more before depressurization, free [O] can be released for a short time by CO deoxidation. It was possible to reduce it to a very low level.

[0016]

As described above, in the present invention, CO deoxidation is actively performed by subjecting medium carbon steel [C] ≧ 0.1% by weight to decompression treatment utilizing CO deoxidation. It is done. Free [O] 3 in a short time due to CO deoxidation
Since it is reduced to an extremely low level of 0 ppm or less, a small amount of Al needs to be added for deoxidation, and an extremely low oxygen molten steel having extremely high cleanliness can be obtained. The obtained high cleanliness molten steel is continuously cast into a slab at a high continuous casting ratio, and the occurrence of rolling defects is suppressed even during hot rolling, cold rolling and the like.

[Brief description of drawings]

FIG. 1 Decompression treatment using a conventional RH degasser

FIG. 2 RH degasser used for depressurization according to the invention

FIG. 3 Relationship between [C] and free [O] during RH degassing process

[Explanation of symbols]

10: Vacuum container 13: Inside of container 30, 32:
Molten steel 40: Oxygen lance 41: Oxygen gas

Claims (2)

[Claims] 1. When the molten steel refined to [C] ≧ 0.10% by weight or more is subjected to a reduced pressure treatment by a vacuum degassing apparatus, it has a pressure of 1330 Pa.
0.03 to 0.3 in molten steel held in the following reduced pressure atmosphere
While supplying oxygen or oxygen-containing gas at a flow rate of 0 Nm ³ / min · ton, depressurize to free [O] ≦ 30 ppm,
A method for producing a high cleanliness steel, which stops the acid transfer when [C] reaches 0.08 to 0.02, and then adds Al. 2. The manufacturing method according to claim 1, wherein molten steel having a [C] before depressurization of 0.15 wt% or more is used.