JPH04157052A - Immersion nozzle for continuous casting and method for preventing nozzle clogging - Google Patents

Abstract

PURPOSE:To reduce defect in a product by providing a porous plug body for blowing gas on the contact surface of a nozzle refractory with a molten steel at a part below an upper end of discharge hole. CONSTITUTION:The porous plug body 11 for gas blowing is provided on the contact surface of nozzle refractory with the molten steel at the part below the upper end position of discharge hole 4 in the immersion nozzle 3 for continuous casting. The insoluble gas seed of Ar, etc., is blown into molten steel from a porous plug body and successively, the soluble gas seed of N2, etc., is blown into the molten steel from the porous plug body. Clogging of the immersion nozzle is prevented by gas blowing. By this method, this can be largely contributed for reduction of unstationary part in a cast slab.

Description

DETAILED DESCRIPTION OF THE INVENTION C. Industrial Application Field The present invention relates to the shape of a submerged nozzle for continuous casting of molten steel and a method for preventing clogging of the submerged nozzle.

f Conventional technology] The prior art will be explained with reference to FIG.

In continuous casting of molten steel, mold 2 is moved from tandem 1 to mold 2.
A submerged nozzle 3 is generally used to inject the molten steel into the molten steel. By the way, during such continuous casting, the molten steel discharged from the immersion nozzle 3 into the continuous casting mold 2 contains non-metallic inclusions (alumina, etc.), and nozzle clogging due to the accumulation of non-metallic inclusions may occur in the nozzle 3. or occur. Conventionally, in order to prevent such nozzle clogging, argon gas is blown into the nozzle 3. The argon gas flowing inside the nozzle cleans the nozzle inner wall and the nozzle discharge port 4, thereby preventing alumina from adhering to these parts and molten steel from forming. Prevents coagulation and adhesion.

The argon gas is normally blown into the tundish upper nozzle 7, the sliding plate 8, or the upper position 9 inside the submerged nozzle, and this gas is blown into the molten steel through a porous plug.

This conventional technique is an extremely effective method for preventing clogging of the nozzle 3 during casting, and is widely used as an excellent technique for stably and long-term continuous casting of aluminum-deoxidized steel. Used industrially.

In the above conventional technology, the gas blown into the nozzle 3 flows into the mold 2 as bubbles 5, and some of them float toward the molten steel scene (meniscus) 6 in the mold due to the difference in specific gravity of the molten steel. The gas is trapped inside the slab]0.The trapped gas causes blisters, which are internal defects, in the subsequent manufacturing process of cold-rolled steel sheets, and if it is trapped on the surface of the slab or under the skin, it will cause damage to the cold-rolled steel sheet. This causes surface defects on the steel plate.

The degree of trapping of the above air bubbles into the slab is greatly influenced by the flow pattern of the injection flow into the mold and the casting speed, and optimization such as changing the shape of the nozzle tip is performed to improve the flow pattern of the injection flow. It is being said. However, when the casting speed is increased with the aim of improving productivity, the amount of air bubbles trapped inside the slab increases, resulting in frequent internal defects, and the problem has not been completely resolved.

[Problems to be Solved by the Invention] As mentioned above, the conventional method has the drawback that air bubbles are trapped in the solidified shell of the slab, which tends to cause internal and surface defects in the product.1 The present invention solves these drawbacks. The present invention has been made in order to provide a submerged nozzle and a method for preventing nozzle clogging, which can overcome this problem and prevent internal and surface defects of products.

[Means for Solving the Problems] The present invention is an immersion nozzle for continuous casting, characterized in that a porous plug body for blowing gas is provided on the surface of the nozzle below the upper end position of the discharge port that contacts the nozzle refractory and molten steel. This is an immersion nozzle for continuous casting. Here, the surface in contact with the nozzle refractory and molten steel below the upper end position of the discharge port refers to the inner surface of the nozzle, the bottom surface of the nozzle, and the inner wall surface of the discharge port, excluding the outer surface of the nozzle. Note that it is preferable that a porous plug body is provided on substantially the entire surface of the nozzle that is in contact with the nozzle refractory and the molten steel.

Further, the method of the present invention is a method using the above-mentioned immersion nozzle, in which an insoluble gas such as argon is blown into the molten steel through the porous plug body, or molten steel such as N2, H2, C3H8, etc. is blown into the molten steel through the porous plug body. This is a method for preventing clogging of a submerged nozzle, which is characterized by blowing a soluble gas into the submerged nozzle.

[Function] The present inventors have conducted many experiments and studies regarding immersion nozzle clogging.

The locations where nozzle clogging occurs are the upper nozzle section, sliding nozzle section, and submerged nozzle discharge port section, where the direction of the flow of molten steel changes. Almost no nozzle clogging was observed in the straight body of the nozzle, that is, in the area where the molten steel flowed in a straight line.

As mentioned above, it is presumed that the gas blown into the nozzle forms a gas film at the interface between the nozzle wall and the molten steel, thereby preventing inclusions from adhering to the nozzle wall.

However, in conventional gas blowing, the gas blowing position is set above the immersion nozzle, so the gas is uniformly dispersed in the molten steel near the nozzle discharge port, and all of the blown gas flows through the nozzle. It could not be said that it effectively contributed to the formation of a gas film at the interface between the wall and molten steel. The gas dispersion behavior in the immersion nozzle was confirmed using a water model with the same dimensions as the actual mold. Additionally, in order to reduce surface and internal defects caused by bubbles in the slab, it was necessary to reduce the amount of blown gas.

In view of these drawbacks, the present invention proposes to provide a gas injection site near the nozzle discharge port, where nozzle clogging is most severe.

An embodiment of the nozzle 3 of the present invention is shown in FIG.

This nozzle 3 is provided with a porous plug body 11 extending from the upper end of the discharge port 4 to the bottom wall of the nozzle, from which gas introduced through a gas blowing introduction path 13 and a gas blowing slit 12 is blown. It's something I'm trying to do. As shown in FIG. 1, (a) nozzle side wall 11, (b) nozzle bottom 14, and (c) discharge port thick part 15 may all be made of porous plug bodies. Also, depending on the situation of nozzle clogging, one or two of the three locations (a), (b), and (c) above may be used; for example, as shown in Figure 2, only the nozzle side wall may be removed. It may also be a porous plug body.

In the method of the present invention, the above-mentioned porous plug body is provided below the upper end of the discharge port of the nozzle, and when blowing gas from this plug body, a gas insoluble in molten steel such as Ar or a gas soluble in molten steel such as N2 is blown. It's crowded. In these cases, nozzle clogging can be prevented with a significantly smaller amount of gas than in conventional gas blowing. Insoluble gas enters the molten steel as bubbles, but the amount is significantly smaller than in the past.

As the soluble gas, N2, N2, C3Hs, etc. can be used alone or in combination.These gases contribute to nozzle clogging at the nozzle discharge port, and are absorbed by the molten steel in the mold, drastically reducing air bubbles in the molten steel. Therefore, there are fewer defects in the product.

[Example 1 Example 1 Under the following casting conditions, strand 1 of the two strands was cast using the conventional method, and strand 2 was cast using the immersion nozzle of the present invention shown in FIG. Regarding strand 2, the Ar gas flow rate from the immersion nozzle was set to 0 to IONβ.
/min.

Continuous casting machine type: 12mH curved continuous casting machine.

2 strand mold shape: 240tx 1600mmW Casting steel type: Ultra low carbon AI2 killed steel (C=20~30ppm, Ajl!=0.03~0.0
5%, T 0 = 40-50 ppm) Throughput: 3
．． 5 t/min Immersion nozzle shape: 8 mm diameter x 2 holes facing downward 10" The experimental conditions are as shown in Table 1.

Figure 4 shows a comparison of the product results of the slabs and the status of nozzle clogging under the above experimental conditions.

In Figure 4, the product defect occurrence index of cold-rolled steel sheets is expressed as an index with the average value of the surface and internal defect occurrence rate of 2.6% of the cold-rolled steel sheet (0.8 mmt) of process Ar-blown strand as 1.0. This is what I did. Further, the nozzle clogging index is similarly calculated as the clogging rate of the nozzle discharge port of the process Ar-blown strand after 5 consecutive times.

Each plot when the Ar flow rate from the immersion nozzle of the strand 2 in FIG. 4 is changed is a plot of the average value of data of 20 to 25 heats (5 heats/1 chance).

The following can be seen from Figure 4.

(b) The Ar gas flow rate from the immersion nozzle is significantly reduced compared to the conventional sliding nozzle (
1/20)l, but the degree of nozzle clogging is smaller than that of the conventional method. This is considered to be because the Ar gas in the discharge port bolus plug body effectively prevents contact between the molten steel and the refractory wall even at a small gas flow rate.

(b) Due to the effect of (a) above, the Ar flow rate can be reduced compared to the conventional method, reducing product defects due to bubbles, and reducing the number of product defects caused by clogging due to nozzle clogging or minor clogging. Defects are also decreasing.

However, the Ar flow rate from the immersion nozzle is 0.5 N I2/
When it becomes less than min, nozzle clogging starts to occur and product defects become worse.

Example 2 Casting was carried out under the same casting conditions as in Example 1, using two strands in the same manner as in Example 1, and under the experimental conditions shown in Table 2.

Strand 1 was produced using the conventional method, Strand 2 was produced using the immersion nozzle of the present invention, and N2 gas soluble in molten steel was used as the blown gas.

Figure 5 shows a comparison of the product results of the slabs and the state of nozzle clogging under the above experimental conditions.

The definitions of the product defect occurrence index, nozzle clogging index, and each plot of the cold-rolled steel sheet in FIG. 5 are the same as those in FIG. 4. The following can be seen from Figure 5.

(a) When N2 is blown from the immersion nozzle outlet, the nozzle clogging is less severe than in the conventional method, even at a low flow rate of N2, as in Example 1.

(b) When N2 gas is used, it is soluble in molten steel, so it effectively acts on clogging the nozzle at the nozzle discharge port, but since it is absorbed by the molten steel in the mold, air bubbles trapped in the slab shell At the same gas flow rate (in the range of 1 to 10 N R/min), the product defect index of the cold-rolled steel sheet is significantly reduced compared to the case of Example 1.

However, if the N2 gas flow rate decreases extremely, (N2≦0.
5Nff/m1n) As in Example 1, nozzle clogging and product defects were comparable to the conventional method.

The above experimental results show that the gas is not limited to N2, but may be N2, C3Ha alone, or a mixture thereof, as long as it is soluble in molten steel.

(Effect of the invention) In this way, a porous plug body is provided near the discharge port of the immersion nozzle for continuous casting, and from this part, an insoluble gas such as Ar gas or a soluble gas such as N2 gas is blown into the molten steel. It was found that by incorporating the method, it was possible to prevent nozzle clogging at a lower flow rate than with conventional methods, and it was also possible to reduce product defects.

Furthermore, by applying the present invention, it is possible to carry out multiple continuous casting using one nozzle, and it is possible to perform continuous casting from 5 to 15 in the conventional method, which greatly contributes to reducing the unsteady parts of the slab.

[Brief explanation of the drawing]

FIG. 1(a) is a vertical cross-sectional view of a submerged nozzle according to an embodiment of the present invention, FIG. 1(b) is a view taken along the line A-A in FIG. 1(a), and FIG. Vertical cross-sectional view of the immersion nozzle of the example, Fig. 2 (
b) is a view taken along the line B-B in Fig. 2(a), Fig. 3 is a schematic diagram of the immersion nozzle section explaining the continuous casting method, and Fig. 4 shows the nozzle clogging index and product defects when using the nozzle of the present invention. FIG. 5 is a graph showing the relationship between the nozzle clogging index, product defects, and N2 flow rate when the nozzle of the present invention is used. 1... Tundish 2... Mold 3... Immersion nozzle 4... Nozzle discharge port 5... Air bubble 6... Meniscus 7... Upper nozzle 8... Sliding plate 9... Immersion Nozzle gas injection position 10...Slab 11...Porous plug body 12...Gas injection slit 13...Gas introduction path 14...Nozzle bottom 15...Discharge port thick part

Claims (1)

[Scope of Claims] 1. An immersed nozzle for continuous casting, characterized in that a porous plug body for blowing gas is provided on the surface of the nozzle lower than the upper end position of the discharge port that contacts the nozzle refractory and molten steel. . 2. An immersed nozzle for continuous casting, characterized in that a porous plug body for blowing gas is provided on substantially the entire surface of the nozzle below the upper end position of the discharge port that contacts the nozzle refractory and molten steel. 3 Ar from the porous plug body according to claim 1 or 2
A method for preventing clogging of an immersion nozzle, which is characterized by blowing an insoluble gas species into molten steel. 4 N_ from the porous plug body according to claim 1 or 2
A method for preventing clogging of an immersion nozzle, characterized by blowing a soluble gas such as 2 gas into molten steel.