JPH0628778B2 - Immersion nozzle for high speed continuous casting - Google Patents

Description

TECHNICAL FIELD The present invention relates to an immersion nozzle suitable for use in high speed continuous casting with a throughput of 3.5 ton / min or more.

(Prior art) Conventionally, in continuous casting, molten steel is supplied without being reoxidized into the mold, and immersion is carried out to promote the floating of inclusions brought into the mold or to prevent breakout during drawing of the slab. Nozzles have been used. For example, in the case of trying to improve the productivity by increasing the molten steel discharge flow rate from 2 to 3 tons / min to 4.0 to 6.0 tons / min in a two-hole type immersion nozzle. It was unavoidable that the mold powder was entangled therein, and it was unavoidable that the cleanliness of the molten steel was significantly reduced.

As for the entrainment of mold powder, the flow velocity at the discharge port of the dipping nozzle is the highest at the lower end, while the flow velocity at the upper end shows a negative flow velocity distribution, and the flow velocity distribution is uneven above and below the discharge port (No. FIG. 3 (a)), and as a prior document relating to this, Japanese Patent Application Laid-Open No. 61-150756 discloses a technique for changing the discharge angle of a nozzle according to the discharge flow rate of molten steel. Japanese Patent Laid-Open No. 65328 discloses a technique of increasing the cross-sectional area of a nozzle outlet to reduce the discharge rate of molten steel.

(Problems to be solved by the invention) By the way, when the discharge angle of the immersion nozzle is changed according to the discharge flow rate of molten steel, there is an advantage that the entrainment of mold powder can be suppressed, but the floating of inclusions in the mold is completely expected. It is unavoidable that it deteriorates the cleanliness of the steel.On the other hand, when enlarging the outlet of the nozzle, the distance from the outlet to the meniscus becomes shorter, so it is unavoidable to entrain the mold powder by the molten steel flow. I want to.

There is an attempt to solve the above-mentioned problem by making the immersion depth of the nozzle deeper than before, but in this case, the flow of the molten steel is below the mold, so there is a high risk of breakout.

It is an object of the present invention to propose a novel immersion nozzle capable of realizing high-speed continuous casting with a throughput of 3.5 tons / min or more without deteriorating the cleanliness of the molten steel injected into the mold.

(Means for Solving the Problem) The present invention relates to an immersion nozzle including a pair of upper and lower discharge ports sandwiching the axis center of the nozzle body in the left and right directions, and each discharge port has a horizontally elongated opening shape and upper and lower discharge ports. Opening distance between outlets l
, Mold length L, 4-hole nozzle throughput y ₄ ,
As the distance Z from the mold upper end up to the meniscus, a immersion nozzle for high-speed continuous casting characterized by comprising a _{l <L-Z-64y 4} -370.

FIG. 1 schematically shows an enlarged vertical cross-section of the tip of the immersion nozzle according to the present invention, in which 1 is a nozzle body,
2 is an upper ejection port, 3 is a lower ejection port, and the upper ejection port 2 and the lower ejection port 3 each have a horizontally long opening shape as shown in FIG. 2) from the upper end of 2 to the lower end of the lower discharge port 3) is defined as the mold length L, the throughput y ₄ in the four-hole nozzle, and the distance Z from the upper end of the mold to the meniscus, 1 <L−Z−64y ₄ −
Become 370.

(Operation) In order to perform high-speed continuous casting with a throughput of 3.5 ton / min or more, even if the molten steel discharge flow rate is increased, the maximum discharge flow rate of the nozzle is not increased as shown in FIG. 3 (b). It is extremely effective to use a 4-hole type immersion nozzle capable of obtaining a uniform flow velocity distribution.

By the way, in such an immersion nozzle, when the height L of the mold is particularly limited as shown in FIG. 4, the distance X from the upper end of the upper discharge port 2a to the meniscus is avoided in order to avoid the occurrence of breakout. Inevitably the mold powder was inhaled because it had to be shortened.

Therefore, in this invention, initial by the respective discharge ports of the upper and lower in the 4 holes type immersion nozzle and having an oblong opening shape, the opening distance l of these discharge ports and _{l <L-Z-64y 4} -370 The purpose was achieved advantageously.

Here the opening distance l of the upper and lower discharge port _{l <L-Z-64y 4} -37
In the ordinary continuous casting (when a two-hole type immersion nozzle is used, refer to FIG. 5), the suction index I of the mold powder is 0, which is the distance X (mm) from the upper end of the discharge port to the meniscus. ) And the throughput y ₂ (ton / min), I = y ₂ −X / 80 (1)

Fig. 6 shows the relationship between the suction index I of the mold powder and the slime extraction amount mg / kg. To ensure the quality of the slab, the slime extraction amount should be 0.03 mg / kg or less, and the suction index I Must be 1.0 or less.

In order to correspond to various throughputs in the above formula (1), the distance X from the upper end of the discharge port to the meniscus
It can be easily dealt with by changing.

On the other hand, as is clear from FIG. 7 which shows the relationship between the distance M from the lower end of the ejection port to the lower end of the mold and the occurrence rate of breakout, particularly when the distance M from the lower end of the ejection port to the lower end of the mold is 450 mm or less. Breakouts occur frequently.

The value of X in this order immersion nozzle, _1> than _y 2 -X / 80, is required to be _{80 (y 2 -1) <X} .

Here, in the case of the 4-hole type immersion nozzle, the throughput of the upper discharge port 2 corresponds to y ₂ , and the maximum discharge flow in the 4-hole type immersion nozzle is about 0.8 times that of the 2-hole type immersion nozzle ( 8th
Therefore, it is necessary to set y ₂ = 0.8y ₄ and the above X should be X> 80 (0.8y ₄ −1) ... (2).

Next, the opening distance l in the four-hole type immersion nozzle is L- (X + Z + l)> 450 from the limit of occurrence of breakout, where L is the length of the mold, where l <L- (X + Z + 450) ... (3 ) Thus opening distance l, from (2) and (3), the _{l <L-Z-64y 4} -370.

It should be noted that the opening shape of the discharge port may be any shape as long as it satisfies the condition of the above-described opening distance l and has a desired discharge flow rate. Further, the discharge angle of the discharge port is preferably 15 to 45 ° downward to prevent reverse flow.

(Example) Using the four-hole type immersion nozzle (nozzle inner diameter d: 70 mm, discharge port shape: square (size 60 × 70 mm)) shown in FIG. 1 above, the component composition was C: 0.0018%, Mn : 0.08%, P: 0.02
Continuous casting of molten steel (extremely low C steel) with 0% and S: 0.015% was performed according to the conditions of Table-1, and the amount of slime inclusions in the molten steel and the suction index of the mold powder at that time were investigated. For comparison, a conventional two-hole type immersion nozzle, 4
The same investigation was conducted for the case of using the hole type immersion nozzle.

The results are also shown in Table 1.

In the continuous casting using the immersion nozzle according to the present invention, very good results could be obtained even though the molten steel throughput was 3.5 tons / min or more.

(Effects of the Invention) According to the present invention, it is possible to obtain a high quality slab without involving mold powder or the like even if the throughput of molten steel is increased in continuous casting.

[Brief description of drawings]

FIG. 1 is an explanatory view of the structure of an immersion nozzle according to the present invention, FIG. 2 is an enlarged view of a main part of the immersion nozzle according to the present invention, and FIGS. 3 (a) and (b) are a two-hole type immersion nozzle and a four-hole type immersion nozzle. Fig. 4 shows a comparison of the flow velocity distribution and discharge rate of molten steel in Fig. 4, Fig. 4 is a vertical sectional view of a 4-hole type immersion nozzle, Fig. 5 is a vertical sectional view of a 2-hole type immersion nozzle, and Fig. 6 is a suction index. Fig. 7 is a graph showing the relationship between I and the amount of slime extracted, Fig. 7 is a graph showing the relationship between the distance M from the lower end of the mold to the lower end of the discharge port and the occurrence rate of breakout, and Fig. 8 is the maximum discharge flow of a conventional immersion nozzle. 7 is a graph comparing the maximum discharge flows of the immersion nozzle according to the present invention. 1 ...... nozzle body 2 ...... on the discharge port 3 ...... under the discharge port, from l ...... opening distance L ...... mold length y ₄ ...... 4 Throughput Z ...... mold the upper end of the hole nozzle up to the meniscus distance

Claims (1)

[Claims] 1. A submerged nozzle comprising a pair of upper and lower discharge ports sandwiching the axis of the nozzle body in the left and right directions, wherein each discharge port has a horizontally elongated opening shape, and an opening distance 1 between the upper and lower discharge ports is a mold. The length L (mm), the throughput in a 4-hole nozzle y ₄ (ton / min), and the distance Z (mm) from the upper end of the mold to the meniscus, l <L−Z−64
Immersion nozzle for high speed continuous casting, characterized by y ₄ -370.