JP2855867B2 - Refining method of chromium-containing molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a refining method for decarburizing molten steel having a high chromium content to an extremely low carbon content.
This method is suitable for producing chromium-containing steel in which the nitrogen content is adjusted to a value within a predetermined range.

[0002]

2. Description of the Related Art The AOD process, which is widely practiced as a method for decarburizing chromium-containing molten steel, is liable to be oxidized when the decarbonization proceeds and the C concentration in the molten steel decreases. Increase the ratio of Ar in the gas to be blown and increase O ₂
Is reduced to reduce the loss of Cr. When the C concentration in the molten steel has dropped to a predetermined value, a reducing agent such as ferrosilicon is charged, only Ar is blown in and stirred, and the chromium oxide in the molten steel generated in the process up to that point is reduced. to recover. In this way, a molten steel whose C content is reduced to a predetermined value and whose Cr content is restored to the level before the refining is obtained.

[0003] However, in the low C region, it takes a long time for the decarburization speed to decrease to reach a desired C concentration,
Oxidation of Cr is easy to proceed. In order to reduce the oxidation of Cr, the ratio of Ar in the blown gas must be increased, but this naturally increases the consumption of Ar and is uneconomical.

[0004] As a measure to promote decarburization in the low C region, there is use of a vacuum refining method. For example,
No. 10087 discloses a method for refining a high chromium stainless steel to a low carbon content of 0.03% or less at normal pressure.
₂ is carried out until C: 0.2-0.4%, after which stirring with non-oxidizing gas is continued but O ₂
Is stopped, and the pressure on the steel bath is continuously lowered to about 10 Torr or less to cause boiling, thereby performing a desired decarburization. JP-A-61-1
The same applies to the refining method described in 36611.
This is a method in which decarburization is performed under atmospheric pressure using a D apparatus, and decarburization is further continued under a reduced pressure of 20 Torr using a vacuum refining apparatus.

The present inventors have found that the carbon concentration is about 0.2% by weight.
To the extent, decarburization by blowing a mixed gas of non-oxidizing gas and oxygen such as argon into high chromium molten steel under atmospheric pressure,
After that, with the refining environment reduced to about 200 Torr or less, decarburizing to a lower concentration by injecting only a non-oxidizing gas such as argon, and oxidizing during the vacuum refining process A method for reducing chromium oxide simultaneously with decarburization by adding a reducing agent to reduce the entire amount of chromium that has been developed has been developed.
-98473.

In the method of the present inventors, when Ar gas is used as the non-oxidizing gas, the N content of the chromium-containing molten steel can be reduced to about 0.02%. In some cases, a higher N content may be more appropriate. The N content is, for example, 0.03-0.
If the predetermined value is to be adjusted within the range of 10%, the blowing of only Ar gas results in an unnecessarily low N content, which necessitates a later nitriding operation, resulting in waste of expensive argon. become.

A method combining the above atmospheric refining and vacuum refining (Japanese Patent Publication No. 60-10087, Japanese Patent Application Laid-Open No. 611-1)
3661), since the supply of O ₂ is stopped from a relatively high C concentration level, the loss due to the oxidation of Cr can be reduced accordingly, but the sudden application of a vacuum causes a large amount of CO gas to be generated, and there is a danger of explosion. Invite. Slow vacuum suction eliminates the danger, but the other problem is that if the elapsed time becomes longer and the steel bath temperature drops, the reaction will slow down. Further, if the pressure is set to a low pressure of 10 Torr or less, there is a problem that the splash of molten steel becomes severe and the hopper for charging the alloy material is blocked. Therefore, in these methods, it is practically impossible to add a reducing agent for recovering oxidized Cr simultaneously with final decarburization. If a reducing agent is added after the decarburization, Cr can be recovered, but the refining time becomes longer.

[0008]

SUMMARY OF THE INVENTION A first object of the present invention is to provide a refining method capable of obtaining a chromium-containing steel having a desired range of nitrogen content by reducing the amount of expensive argon gas used. Is to do.

A second object of the present invention is to promote the decarburization by applying vacuum in the decarburization and refining of chromium-containing molten steel,
Without the danger of an explosion caused by the large amount of gas generated, the splash of molten steel was suppressed to a practically acceptable level, and it was possible to recover Cr by adding a reducing agent simultaneously with the final decarburization. To provide a refining method.

[0010]

The refining method of chromium-containing steel according to the present invention which achieves the above object is a refining method in which gas is blown into molten steel of high chromium steel in a refining vessel to decarburize. A mixed gas of a non-oxidizing gas and O ₂ is used as the blowing gas until the C concentration in the container falls to 0.15% (weight) or less. Is reduced to 150 to 20 Torr, and a reducing agent of a stoichiometric amount or more necessary to reduce chromium oxide in the molten steel is added, and N ₂ or other non-oxidizing gas is introduced into the molten steel. It is characterized by blowing at a flow rate of at least 0.2 Nm ³ / min per ton.

In a preferred embodiment of the present invention, N ₂ is blown as a non-oxidizing gas at the beginning of vacuum refining, and a non-oxidizing gas other than N ₂ such as Ar is blown at a later stage to perform vacuum treatment. Do.

The chromium-containing steel to which the vacuum refining method of the present invention is applied means a steel having a chromium content of 5% or more, and specific examples thereof include a Ni—Cr stainless steel heat-resistant steel and a Cr stainless steel heat-resistant steel.

[0013]

[Action] In general refining, but is expected to nitrogen pressurizing the blowing N ₂ gas is carried out in molten steel, if the nitrogen content of chromium-containing steel is more than 500ppm is, N ₂ gas in a vacuum of less than 200Torr Is blown to perform denitrification.
Therefore, N ₂ can be used instead of expensive Ar gas. Nevertheless, denitrification by blowing N ₂ gas naturally has a certain limit.
After the blowing of the _two gases, it is practically preferable to end the refining operation by blowing the Ar gas. At this time, the timing of switching the blowing gas from N ₂ to Ar may be appropriately controlled so as to obtain molten steel having a predetermined nitrogen content.

One reason for selecting the point at which the refining is changed from atmospheric pressure to vacuum under the condition that the C concentration becomes 0.15% is that if the C concentration in the molten steel becomes lower than 0.2%, O ₂ While decarburization does not proceed effectively,
Loss due to oxidation increases, and the tendency is that C: 0.15%
It becomes even more remarkable. Another reason is that
Operational safety must be ensured. As described above, when vacuum is applied at a level of C concentration of 0.2 to 0.4%, a large amount of CO gas is generated (mainly Cr ₂ O ₃ + C → 3CO ↑ +
2Cr) and may react with O ₂ in the upper space of the smelting vessel or in the exhaust gas duct to explode. Needless to say, explosions must be prevented to ensure worker safety and avoid equipment damage.
With a C concentration of 0.15% or less, the danger of such an explosion is substantially eliminated.

The degree of vacuum must be 200 Torr or less, particularly 150 Torr or less, since the reduction of Cr oxide by the C component in the molten steel, that is, the decarburization and the recovery of Cr are not promoted up to about 300 to 200 Torr. On the other hand,
As described above, excessive vacuum suction causes difficulty in generating splash due to rapid generation of CO, and therefore vacuum suction with an appropriate degree of reduced pressure is performed. The lower limit of 20 Torr has been determined because splash at that pressure remains practically acceptable. Injecting N ₂ or other non-oxidizing gas at a flow rate of at least 0.2 Nm ³ / min per ton of molten steel in this vacuum refining stage is a necessary operation to prevent blockage of the injection nozzle.

[0016]

[Example 1] Using a refining vessel (1) provided with a vacuum hood (4) capable of vacuum suction in a refining furnace having the structure shown in Fig. 1 and using 18Cr-Ni (SUS304). Gas (3) was blown into molten steel (3) as a raw material to perform decarburization refining.

The operation under the atmospheric pressure was carried out for 20 minutes while changing the O ₂ / N ₂ ratio of the blown gas to 6/1, then 3/1, and further to 1/1. Thereby, the C concentration was reduced to 0.15%, and the Cr content was reduced to 17.2%.

Therefore, the stirring gas is changed to N ₂ alone (the flow rate is 0.3 Nm ³ / min. Ton of molten steel), stirring is continued, the container is kept airtight with a lid, and the pressure in the container is reduced to 30 by vacuum suction.
Reduced to Torr.

The time of vacuum refining was set to 10 minutes in total, and the blowing gas was switched from N ₂ to Ar on the way. FIG. 2 shows the entire refining process. Before and after the switching of the blowing gas, ferrosilicon was added to reduce chromium oxide. The addition was made to slightly exceed the stoichiometric amount required to completely reduce the chromium oxide.

The amount of N in the obtained molten steel was measured while changing the Ar blowing time in various ways. The result is shown in FIG. From the graph of FIG. 3, when the blowing of Ar gas is stopped within one minute, the N amount is 500 ppm or more.
400-200ppm less than a minute
It can be seen that the level drops to the level shown in FIG. Therefore, it can be seen that the timing of switching the blowing gas from N ₂ to Ar should be set according to the target N content.

Incidentally, the C content of these steels is 0.01 to 0.1.
05%.

[Embodiment 2] The above 18
In addition to Cr-8Ni steel (SUS304), molten steel as a raw material of 13Cr steel and 24Cr-13Ni steel was used, and decarburization refining was performed under atmospheric pressure in the same manner as in Example 1.

In the subsequent vacuum refining, the blowing gas is changed to N ₂
The timing of switching from Ar to Ar and the amount of Ar gas used during vacuum processing were changed as follows. Switching timing Ar gas consumption A At the start of vacuum suction 2 Nm ³ / ton of molten steel B During vacuum processing 1 C After vacuum processing 0 Obtained when the above three types of raw materials are subjected to vacuum refining of A to C, respectively. The molten steel was measured for N content. The results are as follows (unit is% by weight).

The molten steel was subjected to aspects refining vacuum treatment A B C 18Cr-8Ni 0.024 0.036 0.049 13Cr 0.016 0.024 0.033 24Cr-13Ni 0.036 0.058 0.078

[0025]

According to the refining method of the present invention, since the chromium-containing steel is vacuum-treated using N ₂ gas instead of at least a part of the Ar gas, the amount of expensive Ar gas used can be reduced. Moreover, it becomes easy to adjust the N content of the target chromium-containing steel to an appropriate value.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a vessel during refining for explaining a refining method of the present invention.

FIG. 2 is an explanatory diagram showing a flow of a refining process in the embodiment of the present invention.

FIG. 3 is a graph showing the relationship between the Ar gas blowing time during vacuum refining and the N content in molten steel, which is data of an example of the present invention.

[Explanation of symbols]

 1 Refining vessel 2 Molten steel 3 Gas 4 Vacuum hood

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21C 7/00 C21C 7/068 C21C 7/10

Claims (2)

(57) [Claims] In a refining method in which a gas is blown into molten steel of chromium-containing steel in a refining vessel to perform decarburization, C in the molten steel is removed.
Until the concentration drops to 0.15% (weight) or less, use a mixed gas of a non-oxidizing gas and O ₂ as the blowing gas,
After the C concentration falls below this value, the inside of the container is
While reducing the pressure to 20 Torr, adding a reducing agent in a stoichiometric amount or more necessary to reduce chromium oxide in the molten steel, and adding N ₂ or other non-oxidizing gas to the molten steel at least per ton of the molten steel. A method for refining high chromium steel, characterized by blowing at a flow rate of 0.2 Nm ³ / min. 2. A refining method according to claim 1 Non as the oxidizing gas blowing N _2, at a later stage for performing vacuum processing by blowing a non-oxidizing gas other than N ₂ is at the beginning stage of the vacuum refining.