JPH07166230A - Production of extra-low nitrogen steel - Google Patents

Abstract

PURPOSE:To promote denitriding reaction by CO gas foams caused by carbon (C) in a steel and stably obtain an extra-low nitrogen steel by vacuum degassing treatment. CONSTITUTION:Molten steel 32 for which a converter is brown out to regulate (C) to >=0.10wt.% is subjected to evacuating treatment while it is fed with oxygen 41 at a flow rate of >=0.05Nm<3>/min.ton. The atmosphere of the inside of a vessel 13 is preferably held to a reduced one of <=1330Pa vacuum degree. It is possible to interrupt the top-blow of oxygen at the latter stage of the treatment and to continue the evacuating treatment only. Thus, CO gas foams produced by the decarburizing reaction are utilized for the stirring of the molten steel 32 or the like, by which the extra-low nitrogen and extra-low carbon molten steel or extra-low and low carbon steel in which (N) is regulated to <=20ppm and (C) to <=0.08% can swiftly be obtd.

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 ultra-low nitrogen steel by accelerating denitrification and decarburization of converter steel by decompression treatment using a vacuum degassing device.

[0002]

2. Description of the Related Art Steel materials are used in various fields including those for automobiles, and improvement in workability is required for each application. Workability is improved by reducing impurities such as carbon and nitrogen that cause non-metallic inclusions and hard phases. As a method for reducing carbon in steel (hereinafter, referred to as [C]) and nitrogen in steel (hereinafter, referred to as [N]), various proposals have been conventionally made. A typical example is a decompression process using a vacuum degassing device such as RH and DH. In Japanese Patent Laid-Open No. 4-176812, when decompressing molten steel in which [C] is sprayed to 0.03 to 0.05% by a converter, an oxygen-containing gas is supplied to the molten steel in a vacuum tank to decarburize it. This produces ultra low carbon steel. 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 a descending pipe 12 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 to be treated is charged into a ladle 20. When the lower portions of the ascending pipe 11 and the descending pipe 12 are dipped in the molten steel 30, the ascending pipe 1
The molten steel 30 rises in the inside of 1 and the downcomer 12. Nozzle 14
When an inert gas is blown into the rising pipe 11, the apparent specific gravity of the molten steel in the rising pipe 11 becomes small. Therefore, molten steel 30
Ascending flow 31 rises up the ascending pipe 11, and the vacuum container 10
Sent to. The gas component contained in the molten steel 32 in the vacuum container 10 is contained in the molten steel 32 in the vacuum atmosphere inside the container 13.
Is exposed to the atmosphere from the molten steel 32.
The molten steel 32 containing no gas component has a large specific gravity and is returned from the vacuum container 10 to the ladle 20 as a downward flow 33. Molten steel 30 is ladle 20 → riser pipe 11 → container inside 13
→ repeatedly reflux in order of descending tube 12 → ladle 20, N _2, H
Gas components such as ₂ , O ₂ are removed.

[0004]

When the conventional vacuum degassing in which the depressurization process is started from about [C] = 0.05% by weight, the denitrification rate during the process is small and the denitrification amount is small. This is because, as seen in many reports on basic research, when the molten steel contains a large amount of free [O] and [S], sulfur atoms and oxygen atoms are adsorbed on the molten steel surface,
It is considered that the cause is that the gas-liquid reaction interface area is substantially reduced. The adsorbed sulfur and oxygen atoms are
It works to suppress the denitrification reaction. This tendency becomes a problem when degassing the free molten [O] or [S] converter steel. Specifically, when decarburizing molten steel by converter blowing,
An equilibrium relationship regulated by the CO partial pressure is established between [C] and free [O], and when the end point [C] is reduced, free [O] increases conversely. Free [O]
Increases the adsorbed oxygen, which is a factor that inhibits the denitrification reaction.

As a result, when the molten steel having a low [C] at the start of the vacuum degassing treatment, in other words, the low carbon steel and the ultra low carbon steel which tend to have a high free [O] level, is degassed. The processing time becomes long. In particular, when trying to obtain a steel type required to reduce [N], the denitrification reaction does not proceed sufficiently, and it is the current situation that the target [N] cannot be reduced. The present invention has been devised to solve such a problem, and the [C] of the converter steel to be degassed is set to a high level of 0.10% by weight or more to absorb oxygen. The denitrification reaction in the depressurization process, which tends to decrease the denitrification rate, by reducing the free [O] that is the cause of the above and actively using the CO gas generated by the decarburization reaction for stirring molten steel. The purpose is to rapidly degas molten steel to a target low [N] and low [C] or extremely low [C].

[0006]

In order to achieve the object, the manufacturing method of the present invention, when the molten steel melted in the converter is depressurized by a vacuum degassing apparatus, the carbon content in the molten steel at the time of tapping the converter Oxygen or oxygen-enriched gas was supplied at a flow rate of 0.05 Nm ³ / min · ton or more to the molten steel kept in a reduced pressure atmosphere by setting the concentration to 0.10 wt% or more to remove nitrogen in the steel and carbon in the steel. It is characterized by lowering to 20 ppm or less and 0.08 wt% or less, respectively. The molten steel to which oxygen or oxygen-enriched gas is supplied is preferably exposed to a reduced pressure atmosphere having a vacuum degree of 1330 Pa or less. As the molten steel to be treated, for example, molten steel of [N] ≦ 30 ppm is used. Also,
When [C] is less than 0.05 to 0.02% by weight, the supply of oxygen is stopped, and then the molten steel is depressurized until the carbon concentration in the steel reaches the target concentration. Alternatively, ultra-low nitrogen and ultra-low carbon molten steel can be obtained quickly and efficiently. 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,
From the oxygen lance 40 toward the surface of the molten steel 32, the oxygen gas 4
Spray 1 Oxygen gas 41 is [C] + [O] → C
According to the reaction of O, it reacts with carbon in the molten steel, and CO gas which is a reaction product is released to the inside 13 of the container. Similar CO deoxidation occurs even when an oxygen-rich gas such as an Ar—O ₂ mixed gas is used instead of oxygen. Also, instead of top blowing, side blowing, bottom blowing, or the like, or even when oxygen is supplied in combination with top blowing, similar CO deoxidation proceeds.

[0007]

In the degassing process according to the present invention, [C] is 0.1
Molten steel that is blown down by the converter at 0 wt% or more is used.
In molten steel with a high [C] of 0.10% by weight or more, the free [O] during the depressurization process remains at a low level of less than 50 ppm until the [C] decreases to around 300 ppm. Therefore, the amount of oxygen atoms adsorbed on the surface of the molten steel is quantitatively reduced, and the interfering action of adsorbed oxygen on the denitrification reaction is suppressed. Since the [C] level is high, it is possible to set a large amount of oxygen supply 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. That is, when the [C] of the molten steel to be degassed is 0.10% by weight or more, the denitrification reaction is promoted, and the molten steel having an extremely low nitrogen concentration can be rapidly obtained.

The reduction of free [O] that becomes adsorbed oxygen and the stirring action of molten steel by CO gas bubbles are due to [C] at the time of tapping.
Becomes remarkable when 0.10% by weight or more. [C]
When less than 0.10% by weight, free [O] contained in the molten steel becomes high, and denitrification-inhibiting action by adsorbed oxygen begins to appear. Further, in order to increase the gas-liquid interface area effective for the denitrification reaction, it is necessary to set the oxygen supply rate to 0.05 Nm ³ / min · ton or more. Oxygen supply rate is 0.05
If it is less than Nm ³ / min · ton, CO gas bubbles required for disturbance of the interface and increase of the reaction interfacial area are less generated, and the decarburization rate is reduced. The effect of oxygen supply is vacuum degree of 1330Pa
It becomes remarkable when the molten steel is exposed to the following reduced pressure atmosphere. When the degree of vacuum is 1330 Pa or less, the concentration of oxygen adsorbed on the surface of the molten steel is reduced, and the CO gas generated by the decarburization reaction is easily released into the atmosphere.
Deoxidation is promoted. Along with this, nitrogen [N] in steel is rapidly removed.

When decompressing under such conditions,
Up to about 0.05 to 0.02% by weight of [C], there is substantially no adverse effect due to free [O], so it is preferable to carry out degassing while continuing the supply of oxygen. However, if the supply of oxygen is continued in the latter stage of the treatment in which [C] is less than 0.05 to 0.02% by weight, the amount of free [O] taken into the molten steel increases, and the adverse effect of adsorbed oxygen tends to appear. In such a case, by stopping the oxygen supply in the latter stage of the treatment, denitrification has progressed sufficiently by then, and the actual detrimental effect due to the decrease in the denitrification rate in the latter stage of the treatment, which results in low carbon and high free [O], occurs. It is avoided and ultra low nitrogen / low carbon steel or ultra low nitrogen / ultra low carbon steel is melted with high productivity.

[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. In Example 1, [C] = 0.
The molten steel 30 obtained by refining to 19 wt% and free [O] = 71 ppm was charged into the ladle 20. Next, after immersing the ascending pipe 11 and the descending pipe 12 of the vacuum container 10 in the molten steel 30, the inside 13 of the container is maintained at a vacuum degree of 500 Pa,
Oxygen 41 was blown from the oxygen lance 40 onto the surface of the molten steel 32 for 13 minutes at a flow rate of 0.09 Nm ³ / min · ton. Then, the supply of oxygen was stopped, and the pressure was reduced at 40 Pa for 10 minutes. Furthermore, an Al-based deoxidizer and auxiliary raw materials for component adjustment were added, and the melting process was completed. The operating specifications at this time are shown in Table 1 in comparison with Example 2 and Comparative Examples 1 to 3.

[0011]

[Table 1]

[N], [C] and free [O] of each molten steel
Changed from the start to the end of the RH degassing process as shown in FIGS. Table 2 shows the [N] of the molten steel before and after the treatment for each depressurization treatment.

[0013]

[Table 2]

In Example 1, [N] which exceeded 20 ppm at the start of the treatment decreased with the progress of decarburization as shown in FIGS. 3 and 4, and reached 20 ppm in about 13 minutes.
As shown in FIG. 4, [C] drastically decreased due to the supply of oxygen, and finally reached 21 ppm. Free [O]
As shown in FIG. 5, although it remained at a low level of less than 40 ppm in the first half of the treatment, [C] was 0.05% by weight.
Started to rise when it dropped to 400p in the latter half of the treatment
It was over pm. However, since [N] had already reached 11 ppm at the early stage of the treatment with oxygen supply,
The inhibitory effect of oxygen on the denitrification reaction could be substantially avoided. As a result, [N] was extremely low at 11 ppm,
[C] = 0.0021% by weight of ultra low nitrogen and ultra low carbon steel was obtained.

In Example 2, [C] = 0.169% by weight
The molten steel that had been blown down by the converter in step 1 was subjected to a pressure reduction treatment at a vacuum degree of 500 Pa for 16 minutes while being blown with oxygen at a flow rate of 0.06 Nm ³ / min · ton. In this case, the denitrification rate was slower than that in Example 1, and it took 15 minutes from the start of treatment until [N] reached 15 ppm. However, it was finally denitrified to [N] = 13 ppm. Due to the lower oxygen supply rate compared to Example 1, the decarburization rate was rather slow, taking 21 minutes to reach the low carbon level of [C] = 0.061 wt%. Free [O] remained at a level below 100 ppm throughout the entire treatment period. Although the processing time is long according to the second embodiment,
A very low nitrogen, low carbon steel was obtained.

In Comparative Example 1, as in the case of melting ordinary ultra-low carbon steel, decarburization was performed in a converter to a low carbon level of 0.043% by weight, followed by using an RH degasser. This is an example of a reduced pressure treatment. In this case, since the free [O] at the start of the treatment was as high as 680 ppm, decarburization proceeded rapidly to an extremely low carbon level, and [C] reached 9 ppm at the treatment time of 17 minutes. However, the denitrification rate was the slowest, and [N] at the final treatment showed a relatively high value of 19 ppm, and the denitrification amount was 3
It remained at ppm. In Comparative Example 2, [C] = 0.179% by weight and free [O] = 50 pp when the converter was tapped.
The molten steel of m was subjected only to the depressurization treatment at the ultimate vacuum degree of 470 Pa. The decarburization rate was slower than that of the example, and [C] remained at 0.13% by weight in the treatment time of 18 minutes. The denitrification reaction also proceeded slowly, and the final [N] reached 20 ppm. On the other hand, free [O] was at a low level of 30 to 40 ppm just before deoxidation.

In Comparative Example 3, after the steel was taken out from the converter with [C] = 0.165% by weight, the RH degassing treatment was continued for 24 minutes while supplying oxygen at a flow rate of 0.03 Nm ³ / min · ton. Further, it is an example in which the top blowing of oxygen was stopped and the pressure reduction treatment was performed. In this case, since the oxygen supply was 1/3 of that in Example 1, the decarburization rate was slow, and it took 24 minutes from the start of the treatment until [C] reached 0.05% by weight. The decarburization reaction proceeded to an extremely low carbon level with the subsequent degassing treatment, and the final [C] reached 23 ppm. The denitrification rate was slow as compared with the examples, and the reached [N] was only 17 ppm.
From this, it was confirmed that a sufficient decarburizing effect cannot be obtained with a small oxygen supply rate as in Comparative Example 3. As is clear from this comparison, [C] at the start of degassing treatment
Of 0.10% by weight or more, and when a molten steel held under reduced pressure is blown with oxygen at a flow rate of 0.05 Nm ³ / min · ton or more, [C] is effectively consumed in the decarburization reaction and is generated accordingly. It can be seen that the stirring of molten steel is activated by the CO gas bubbles that are generated. As a result, the reaction interfacial area effective for denitrification increases under the condition that the effect of adsorbed oxygen, which is detrimental to the denitrification reaction, is eliminated. It became possible to lower it. Moreover,
It has become possible to set [C] and free [O] to a low level or an extremely low level.

[0018]

As described above, according to the present invention, in the molten steel which is blown by the converter so that the carbon concentration [C] in the steel becomes 0.10% by weight or more, 0.05 Nm ³ / min. The pressure is reduced while supplying oxygen at a flow rate of at least tons. The supplied oxygen actively reacts with carbon in the steel, and CO gas bubbles as a reaction product vigorously stirs and splashes the molten steel. Thereby, the interfacial area of the molten steel effective for the denitrification reaction becomes large, and the molten steel is quickly denitrified. In addition, since [C] is high, the amount of free [O] contained in the molten steel to be depressurized is small, and the influence of adsorbed oxygen harmful to the denitrification reaction is suppressed. As described above, according to the present invention, the extremely low nitrogen / low carbon steel or the extremely low nitrogen / ultra low carbon steel of [N] ≦ 15 ppm is stably manufactured.

[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] Change in [N] of each molten steel during RH degassing

FIG. 4 [C] change of each molten steel during RH degassing process

FIG. 5: Free [O] change of each molten steel during RH degassing process

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

Claims (4)

[Claims] 1. When the molten steel produced in the converter is decompressed by a vacuum degassing apparatus, the carbon concentration in the molten steel at the time of tapping the converter is 0.1.
Oxygen or oxygen-enriched gas is supplied to molten steel held in a reduced-pressure atmosphere at a flow rate of 0.05 Nm ³ / min · ton or more, and the content of nitrogen in steel is 20 ppm or less and carbon in steel is 0.08. A method for producing ultra-low nitrogen steel in an amount of less than or equal to% by weight. 2. The manufacturing method according to claim 1, wherein the molten steel is exposed to a reduced pressure atmosphere having a vacuum degree of 1330 Pa or less. 3. The production method according to claim 1, wherein the nitrogen concentration in the molten steel before vacuum degassing is in the range of 10 to 30 ppm. 4. Carbon in molten steel is 0.05 to 0.02% by weight.
Supply of oxygen or oxygen-enriched gas is stopped when
The method according to claim 1, wherein the molten steel is subsequently depressurized until the carbon concentration in the steel reaches a target concentration.