JP4412240B2 - Gas blowing nozzle and method for producing ultra-low nitrogen steel - Google Patents

Description

  The present invention uses a vacuum degassing apparatus used in a steel refining method, a gas blowing nozzle capable of melting ultra-low nitrogen steel stably at low cost, and an extremely low level using this gas blowing nozzle. The present invention relates to a method for producing nitrogen steel.

  As a steel refining method, there is a method called RH in which ultra-low nitrogen steel is produced by degassing by immersing two dip tubes installed in the lower part of a vacuum chamber in molten steel in a ladle. However, in this RH method, the nitrogen concentration in the steel may increase during the degassing process, which is a problem.

  It is considered that the increase in the nitrogen concentration in the steel during the degassing process is caused by the suction of air around the dip tube from a connection flange of the dip tube or a crack generated in the dip tube.

Therefore, Patent Document 1 proposes a technique for melting ultra-low nitrogen steel by performing degassing while blowing gas at a speed higher than the speed of sound around the descending dip tube of the RH vacuum degassing apparatus. .
JP-A-3-61310

Also, after the vacuum degassing tank and ladle are connected and sealed, the space created by the vacuum degassing tank and ladle is pressurized with Ar gas, Ar gas is blown in the vacuum degassing tank, or the carbon source is turned on. Patent Document 2 proposes a technique for melting ultra-low nitrogen steel by spraying.
JP-A-6-145769

  However, in the technique proposed in Patent Document 1, since the atmosphere just outside the blowing gas flow is the atmosphere, the entrainment of the atmosphere occurs when the gas is blown at a high speed, and the gas entering from the dip tube contains nitrogen. It will be. Therefore, when the vacuum degassing treatment time is short, nitrogen absorption does not appear apparently, but when the treatment time becomes long by performing decarburization treatment or heat treatment in addition to degassing, There was a problem that the nitrogen concentration of the liquid crystal increased.

  Further, the technique proposed in Patent Document 2 has the following problems in actual operation. That is, during the degassing operation by the vacuum degassing apparatus, it is necessary to constantly monitor the immersion depth, the molten metal surface state, and the slag state of the dip pipe. Although it is necessary to collect a molten steel sample for the above, these operations are impossible when the vacuum degassing tank and the ladle are connected and sealed.

  In addition, the relative positional relationship between the ladle and the vacuum degassing tank needs to be appropriately changed depending on the amount of molten steel in the ladle and the degree of vacuum in the vacuum degassing tank being processed. In order to make the necessary equipment compatible with them, an advanced equipment structure and control system are required, which increases the cost.

  The problem to be solved by the present invention is that the conventional method for producing ultra-low nitrogen steel by the RH method cannot effectively suppress the increase of nitrogen concentration in the steel due to air intrusion, and stable refining of ultra-low nitrogen steel. Is that it could not be carried out inexpensively.

The gas blowing nozzle of the present invention is
In the RH method, in order to suppress an increase in nitrogen concentration in steel due to air intrusion,
Two dip tubes installed in the lower part of the vacuum chamber are immersed in the molten steel in the ladle, and gas is blown from one of the dip tubes so that the molten steel is circulated between the vacuum chamber and the ladle and degassed. In producing a very low nitrogen steel by performing a gas blowing nozzle for purging an inert gas around the dip tube during the degassing process,
The main feature is that a rectifying plate is provided at the gas outlet of the gas blowing nozzle to prevent air from being caught from the opening between the vacuum chamber and the ladle.

In the gas blowing nozzle of the present invention,
The rectifying plate is formed of a horizontal plate and a vertical plate suspended from the outer peripheral end of the horizontal plate,
The outer peripheral radius (m) of the horizontal plate is obtained by multiplying the inner peripheral surface radius (m) of the gas blowing nozzle by the conversion coefficient 2 to the outlet flow velocity (m / s) of the gas blown from the gas blowing nozzle. Have a size greater than or equal to
Further, the height (m) of the vertical plate is equal to or greater than the outer peripheral radius of the horizontal plate (circular radius of the same area when the horizontal plate is not circular) (m), or the molten steel surface and the gas blowing nozzle the smaller of the half of the distance between the gas ejection port, the range of providing a vertical plate (m) is 1/3 or more of the outer periphery of the horizontal plate, so that the 1/2 hereinafter, collected and vacuum tank Entrainment of air from the opening between the pots to the periphery of the gas ejection port can be effectively prevented. Needless to say, the conversion coefficient 2 has dimension (s).

  The outer peripheral radius of the horizontal plate, the height of the vertical plate, and the range in which the vertical plate is provided in the gas blowing nozzle of the present invention have been determined based on numerical analysis by the inventors and experimental results. That is, if the outer radius of the horizontal plate, the height of the vertical plate, and the range in which the vertical plate is provided deviate from the above range, a vertical flow that merges with the purge gas flow can be formed on the vertical plate side from the axial center of the rectifying plate. This is because it will draw in outside air.

  Using the gas blowing nozzle of the present invention, an inert gas is purged around the dip tube during the degassing process. More specifically, the gas blow nozzle is arranged so that the inner surface of the vertical plate faces the dip tube. If the inert gas is purged around the dip tube, the degassing process can be performed while effectively preventing air entrainment from the opening between the vacuum chamber and the ladle. This is the method for producing the ultra-low nitrogen steel of the present invention.

  By this invention, the raise of the nitrogen concentration in steel in RH method can be suppressed effectively, and there exists an advantage that the stable refining of ultra-low nitrogen steel can be performed at low cost.

  The applicant stated that the conventional method for producing ultra-low nitrogen steel by the RH method could not suppress an increase in nitrogen concentration in the steel due to air intrusion, and could not perform stable refining of ultra-low nitrogen steel at low cost. In order to solve the problem, Japanese Patent Laid-Open No. 10-140228 has been proposed.

  In the method proposed by the present applicant, the exposed portion of slag or molten steel between the dip tube and the ladle is covered with a gas seal plate to form a space portion, and degassing is performed while supplying an inert gas to the space portion. The processing is performed.

  However, when the inventors proceeded with experimental studies on the method proposed by the present applicant, there were cases where the effect was unstable. Therefore, as a result of comparing the test situation with the occurrence of the effect, it has been found that outside air (air) may enter from the opening between the gas seal plate and the ladle.

In this case, as shown in FIG. 4, since the flow by the purge gas (Ar, CO _2, etc.) blown from the gas blowing nozzle 1 entrains the outside air (air) that has entered, the purge situation around the dip tube 2 is It can be inferred that the purge effect becomes unstable due to insufficiency. In addition, 3 in FIG. 4 shows a vacuum chamber and 4 shows molten steel.

  As a result, the idea that stable purge around the dip tube would be possible if the entrainment of outside air from the direction of the opening by the purge gas blown from the gas blowing nozzle provided around the dip tube could be suppressed. Note that such a problem does not occur when the sealed structure as in Patent Document 2 described above is used, but in that case, a large-scale and precise structure is required, resulting in an increase in cost.

  The inventors analyzed the purge situation around the dip tube by numerical calculation and confirmed that the inferred phenomenon occurred. Further, the shape of the gas blowing nozzle capable of preventing the entrainment is studied, and the sealing structure shown in Patent Document 2 and the nozzle shape which does not require the installation of the gas seal plate in the method proposed by the present applicant are devised. It was established.

The present invention has been made on the basis of such an idea and examination by the inventors. Hereinafter, the best mode of the gas blowing nozzle of the present invention will be described with reference to FIGS.
3, for example, as shown in FIG. 3, the present invention has eight inner diameters of 50 mm and an outer diameter of 60 mm, which are arranged at equidistant positions just below the bottom of the vacuum chamber 3 around 20 cm from the outside of the dip tube 2. The gas flow nozzle 11 is provided with a rectifying plate 12 formed by a horizontal plate 12a and a vertical plate 12b suspended from the outer peripheral end of the horizontal plate 12a.

  The horizontal plate 12 a has an outer peripheral radius R (m) equal to an inner peripheral radius r (m) of the gas blowing nozzle 11, and an outlet flow velocity (m / s) of the gas blown from the gas blowing nozzle 11. For example, a ring shape having an outer radius of 0.15 m and an inner radius of 50 mm.

  The vertical plate 12b has a height h (m) equal to or greater than the radius R (m) of the horizontal plate 12a, or the distance between the molten steel 4 and the gas outlet 11a of the gas injection nozzle 11. The smaller of 1/2 of L, for example, 0.15 m.

  The vertical plate 12a having such a height is not provided over the entire outer periphery of the horizontal plate 12a, but only within a range (m) of 1/3 or more and 1/2 or less of the outer periphery of the horizontal plate 12a. As shown in FIG. 2, the ½ circumference is provided so that the inner circumferential surface faces the dip tube 2.

  In the production of extremely low nitrogen steel by the RH method using the gas blowing nozzle 11 of the present invention as described above, the gas flow can be controlled as shown in FIG. That is, if the inert gas is purged around the dip tube 2 from the gas blowing nozzle 11 of the present invention, a circulating flow (arrow A) is formed from the axial center of the rectifying plate 12 (in the dip tube 2 side). On the other hand, on the vertical plate 12b side from the axial center of the rectifying plate 12, a vertical flow (arrow B) is formed by the effect of the vertical plate 12b to join the purge gas flow.

  Therefore, in the manufacturing method of the ultra low nitrogen steel of the present invention by the RH method using the gas blowing nozzle 11 of the present invention, the outside air entrained in the purge gas flow is minimized and the opening between the vacuum chamber 3 and the ladle 5 It is possible to effectively prevent air entrainment from 6 and to suppress the influence on the increase in the nitrogen concentration in the steel to a level that can be virtually ignored.

Hereinafter, an example of the results of experiments conducted to confirm the effects of the present invention will be described.
The experiment was performed using about 300 tons of molten steel roughly decarburized in a converter. Al was added at the time of discharging the molten steel to the ladle, but when performing vacuum decarburization with RH, the steel was discharged without deoxidation. This ladle was transferred to RH, and processing was started immediately. Simultaneously with the start of the RH treatment, 0.5 Nm ³ / hr Ar gas was blown perpendicularly toward the molten steel surface from each of the gas blowing nozzles described above to perform a purge.

As a comparative example, a case using a gas blowing nozzle in which a rectifying plate is not provided at the gas outlet of the gas blowing nozzle described above (Comparative Example 1) and a case where normal processing is performed without performing any purging (Comparative Example 2) ) Under the same conditions.
The treatment time was 13 to 30 minutes, and the degree of vacuum was 133 Pa to 1330 Pa.

The N concentration before the treatment and the N concentration after the treatment were analyzed to determine the nitrogen absorption and denitrification rates. The denitrification rate was calculated by the following formula.
Denitrification rate = (N concentration before treatment−N concentration after treatment) / treatment time N concentration before treatment and after treatment was ppm, treatment time was minutes, and denitrification rate was ppm / min.

In the case of denitrification, ie, N concentration before treatment> N concentration after treatment, the denitrification rate is positive, and in the case of nitrogen absorption, ie, N concentration before treatment <N concentration after treatment, the denitrification rate is negative.
The molten steel main components and denitrification rates used in the inventive examples and the comparative examples are shown in Table 1 below.

From Table 1, it is clear that a large denitrification rate can be obtained in the invention examples regardless of the steel type.
On the other hand, in Comparative Example 1, the denitrification rate may be positive, but it may be negative, indicating that the performance is not stable. Moreover, in the comparative example 2, all the denitrification speed became negative, and all nitrogen absorption advanced.

  The present invention is not limited to those shown in the above embodiments, and it goes without saying that the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

  For example, in the above embodiment, the horizontal plate forming the rectifying plate has been described as having a ring shape, but other than the ring shape may be used. However, when the horizontal plate has a shape other than the ring shape, the radius of the horizontal plate used for obtaining the height of the vertical plate is a circular radius of the same area.

  The present invention is not limited to the above example, and may be used for other applications as long as purge is performed while preventing the entrainment of outside air as in the present invention.

It is a figure explaining the baffle plate of the gas blowing nozzle of this invention, (a) is sectional drawing seen from the front, (b) is the figure seen from the bottom face. (A) is the figure which showed the arrangement position of the gas blowing nozzle of this invention from the vertical cross-section direction, (b) is the figure which showed the image of the rectification effect. It is the figure which showed the arrangement position of the gas blowing nozzle of this invention from the horizontal cross-section direction. It is the figure which showed the image of the flow of purge gas at the time of using the gas blowing nozzle without a baffle plate.

Explanation of symbols

2 Immersion tube 3 Vacuum tank 4 Molten steel 5 Ladle 6 Opening 11 Gas blowing nozzle 11a Gas outlet 12 Current plate 12a Horizontal plate 12b Vertical plate

Claims (2)

Two dip tubes installed in the lower part of the vacuum chamber are immersed in the molten steel in the ladle, and gas is blown from one of the dip tubes so that the molten steel is circulated between the vacuum chamber and the ladle and degassed. In producing a very low nitrogen steel by performing a gas blowing nozzle for purging an inert gas around the dip tube during the degassing process,
A gas flow outlet of the gas blowing nozzle is provided with a rectifying plate for preventing air entrainment from the opening between the vacuum chamber and the ladle ,
The rectifying plate is formed of a horizontal plate and a vertical plate suspended from the outer peripheral end of the horizontal plate,
The outer peripheral radius (m) of the horizontal plate is obtained by multiplying the inner peripheral surface radius (m) of the gas blowing nozzle by the conversion coefficient 2 to the outlet flow velocity (m / s) of the gas blown from the gas blowing nozzle. Have a size greater than or equal to
Further, the height (m) of the vertical plate is equal to or greater than the outer peripheral radius of the horizontal plate (circular radius of the same area when the horizontal plate is not circular) (m), or the molten steel surface and the gas blowing nozzle The range (m) in which the vertical plate is provided on the smaller one-half of the distance to the gas jetting port is 1/3 or more and 1/2 or less of the outer periphery of the horizontal plate. nozzle. Two dip tubes provided at the bottom of the vacuum tank are immersed in the molten steel in the ladle, gas is blown from one of the dip tubes, and the molten steel is circulated between the vacuum tank and the ladle to degas. When producing ultra-low nitrogen steel,
During the degassing treatment, electrode gas blowing nozzle according to claim 1, the inner surface of the vertical plate installed to face the dip tube, characterized by purging an inert gas around the immersion tube Low nitrogen steel manufacturing method .

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