JPH01157751A - Submerged nozzle for continuous casting - Google Patents

Abstract

PURPOSE:To improve the quality of a cast slab by reducing the cross sectional area of molten steel passage in a submerged nozzle toward the lower part, forming discharging holes at the upper part and the lower part thereof and specifying the cross sectional area of the discharging holes and the molten steel passage. CONSTITUTION:The cross sectional area for the molten steel passage 3 in the submerged nozzle 1 is formed as reducing to the sectional areas S1, S2...Sn in order from the upper part and the discharging holes 2 having the cross sectional area s1, s2...sn are arranged in order at upper part and lower part of each reduced part. Then, coefficient of discharge K is set to 0.7<=K<=1.0 and the cross sectional area S1-Sn for each passage 3 and the cross sectional area s1-sn for each discharging hole 2 are set so as to satisfy each equation based on the consecutive equation and Bernoulli's equation. By this method, as the discharging speed of the molten steel from each discharging hole s1-sn in the submerged nozzle 1 is almost uniformized, the invading depth of the molten steel flow into the inner face in the cast slab is made to shallow and the enclosure of the inclusion is reduced. Therefore, the quality of the cast slab is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an immersion nozzle used for manufacturing steel, particularly for continuous casting of clean steel with few powdery inclusions and oxide nonmetallic inclusions.

<Conventional technology> Conventionally, during continuous casting of steel, it has been a problem that oxide-based nonmetallic inclusions contained in the poured molten steel are deeply engulfed inside the slab by the poured molten steel flow. In a curved continuous casting machine, the non-metallic inclusions that are deeply rolled up do not float up to the meniscus area and are caught on the lower surface of the solidified shell, forming a sliver on the surface of the steel plate after rolling.
A problem has arisen in which defects such as blisters occur.

Japanese Patent Application Laid-Open No. 61-235 as a technology to solve the above problems.
No. 58 (prior art 1) and Utility Model Application Publication No. 55-88347 (prior art 2) disclose a technique for improving a submerged nozzle to prevent the molten steel flow from entering the unsolidified region.

The immersion nozzle according to the prior art l shown in FIG. The tip of the slurry is curved into a hemispherical shape, and the molten steel flows out from three or more discharge holes provided therein. Also the 6th
The immersion nozzle shown in rjgJ has two discharge holes that are opened diagonally downward and directly above two discharge holes that are opened horizontally or diagonally upward in opposite directions at the lower end of the nozzle. It is intended to cause a collision.

The problem with these nozzles is that when the flow velocity of the molten steel inside the nozzle increases, the molten steel flows out only from the discharge hole at the bottom end, instead promoting a fast downward flow and increasing the penetration depth of the molten steel. There is.

<Problems to be Solved by the Invention> The present invention solves the problem that with conventional immersion nozzles, the penetration depth of the molten steel flow into the inside of the slab is deep, making it difficult to completely prevent nonmetallic inclusions from being entrained. Therefore, this method was developed in order to provide a submerged nozzle that uniformizes the outflow velocity from the discharge hole, promotes the floating of bubbles and nonmetallic inclusions accompanying the flow of molten steel, and produces slabs with fewer defects.

<Means for Solving the Problems> As a result of extensive research into making the flow rate uniform from the discharge holes of the immersion nozzle, the present inventors discovered that a plurality of discharge holes are arranged symmetrically and vertically. The present inventors have found that the objective can be achieved by providing a molten steel passage of the immersion nozzle and making the lower part of the molten steel passage smaller. Based on this knowledge, the present invention has been completed.

The present invention is an immersion nozzle in which a plurality of discharge ports are symmetrically provided in the height direction of a bottomed cylindrical nozzle, and the cross-sectional area of the molten steel passage passing through the immersion nozzle in the height direction is narrowed toward the bottom. , the discharge holes are opened at the top and bottom of the narrowed part, and the cross-sectional area of each discharge hole (31 + s wealth from the top, ・
!11-1+s, ) and the cross-sectional area of the molten steel passage corresponding to each discharge hole (Sl+S2...5II-8 in order from the top)
This immersion nozzle for continuous casting is characterized by satisfying the following relationship regarding S11).

・・・・・・−・・・・・・・・・ is)0.7≦K≦
1.0 Effect〉.

An example of the shape of the submerged nozzle according to the present invention is shown in FIG.

The reason for choosing this shape will be explained below.

When the discharge holes are installed in the vertical direction of the immersion nozzle, the molten steel flow does not necessarily flow uniformly from each discharge hole due to the relationship between the discharge hole area and the molten steel passage cross-sectional area.

When molten steel flows out only from the lower discharge hole, it becomes a fast downward flow and penetrates deep into the slab. Also, when molten steel flows out only from the upper discharge hole, the scene changes rapidly and powder is not entrained. In order to prevent this, it is important to allow Wjtm to flow out from each discharge hole at a uniform speed.

As a result of the research conducted by the present inventors, the unbalance of the molten steel discharge flow from the upper discharge hole and the lower discharge hole in a cylindrical submerged nozzle is due to the fact that the static pressure is small in the upper part, where the flow velocity inside the nozzle is faster, according to Bernoulli's law. It turned out that it was caused by something. Therefore, it has been found that the balance of the molten steel discharge flow can be achieved by making the lower part of the immersion nozzle narrower.

Here, the conditions for the discharge hole area and how to narrow the molten steel passage will be explained below.

FIG. 2 shows the relationships among the molten steel passage area, discharge hole area, and molten steel flow rate in the immersion nozzle of the present invention, respectively, using symbols. Note that the driving force for outflow from the upper discharge hole is due to the dynamic pressure generated in the constriction section.

In the case of two stages of discharge holes; Continuity equation 'KSzUz = 2's, [1---
-(2) Bernoulli's formula % formula % (4) In the case of three stages of discharge holes; From the continuous formula Bernoulli's formula (6) to C... 031 The discharge hole is determined by the above formula. Area and molten steel'i! The relationship between i-road areas is determined.

Although it is possible to have four or more stages of ejection holes, it is best to have two to three stages of ejection holes because the upper ejection holes may approach the meniscus and increase scene fluctuations.

Here, ni' is the runoff coefficient. Strictly speaking, the value of ni' in each outflow section is different, but approximately, the vertical runoff coefficient is K, and the horizontal runoff coefficient is K. ' (this is eliminated in the process of deriving the formula and has no practical effect), and can be assumed to be constant.

As mentioned above, K is the runoff coefficient, which empirically has a value of about 0.8. The cross section of each channel is 0.

In practice, it is permissible to deviate somewhat from the ideal conditions that satisfy the opening formula. If we redefine the tolerance coefficient (×K) as K, which is expressed by including the range of the cross-sectional area to be punched in practical use, then the condition where K is 1 or less and 0.7 or more is the preferred tolerance range in the present invention. I can say that. The appropriate range indicated by diagonal lines in Figure 3 is K
This shows the relationship between the discharge hole area and the molten steel passage cross-sectional area ratio so that the ratio is 0.7 or more and 1.0 or less. , the molten steel passage cross-sectional area ratio may be set.

When there are two discharge holes, the discharge hole area and the molten steel passage area are limited in advance by the size of the nozzle, so if Sl is determined within an allowable range, 38 is calculated.

Similarly, if there are three discharge holes, the area of the three discharge holes! ++
s Yi, 3. First, the area ratio of the lower two stages of is determined, and if S is determined according to the nozzle size, S□
is calculated. Also, the size of S was calculated by the following calculation: Sl+5! + s3+ S
It is obtained by substituting l+sx.

The range of the discharge hole area ratio (upper/upper/lower) and the molten steel passage area ratio (lower/upper) that makes the upper and lower discharge flow velocity uniform, calculated from this formula, is the range sandwiched between the solid lines in Figure 3. . However, as a result of verification using a water model, when the upper or lower discharge holes become significantly smaller, drifting and an increase in the negative pressure area occur, resulting in a discharge hole area ratio (upper/upper/lower) of 0.2 to 0. 8, the uniformity of the flow rate could not be maintained. Therefore, the appropriate range is the shaded range in FIG. Also shown in FIG. 3 are contour lines of the maximum flow velocity ratio in the upper and lower discharge holes. The shaded area is approximately the maximum flow velocity ratio of 1.
Within 4.

Using each nozzle with a discharge hole area 1.7 times that of a normal nozzle and a maximum flow velocity ratio of 1.0 to 1.9 between the upper and lower discharge holes, casting was performed at a casting speed of 1.5 m/sin, and the slab was cast. Figure 4 shows the evaluation of inclusions detected inside.

When the maximum flow velocity ratio becomes 1.4 or more, inclusions increase. In addition, the inclusion rating for the normal nozzle was 5.0.

<Example> A submerged nozzle according to the present invention with two stages of discharge holes was manufactured based on equation (5), and the output was 2.5 t/sin.4. In the case of a hot water supply amount of Ot/sin, the flow rate ratio at each discharge hole was measured in advance using a pitot tube using a water model, and samples were taken from the slab of the heat to evaluate inclusions.

At the same time, regarding the conventional immersion nozzle as shown in Figure 6,
As a comparative example outside the preferred range, it was cast under the same conditions and similarly measured and evaluated, and the results are shown in Table 1.

As shown in Table 1, by using the immersion nozzle of the present invention, the inclusion score was halved, and it is clear that the use of the immersion nozzle of the present invention is effective in improving the quality of finished products.

Table 1 <Effects of the Invention> By using the immersion nozzle of the present invention, the penetration depth of the molten steel flow into the slab can be reduced, the amount of nonmetallic inclusions caught in the slab can be reduced, and the The quality of the pieces was improved.

[Brief explanation of the drawing]

FIG. 1(a) is a front view of the immersion nozzle of the present invention, FIG. 1(b) is a side view thereof, and FIG. 2 is an explanatory diagram of the discharge hole area and molten steel passage area of the immersion nozzle of the present invention. Figure 3 shows the discharge hole area ratio and f! Jf/! FIG. 4 is a characteristic diagram showing the appropriate range of the four-passage area ratio; FIG. 4 is a characteristic diagram showing the relationship between the maximum flow velocity ratio of the submerged nozzle and the inclusion score; FIG. 5(a) is a front sectional view of a conventional nozzle; FIG. 5(b) is a side view thereof, and FIG. 6 is a front sectional view of the conventional nozzle. l... Immersion nozzle, 2... Discharge hole, 3... Molten steel passage. Patent applicant: Kawasaki Steel Corporation Figure 1 (a) (b) Figure 2 In the case of two stages of discharge holes In the case of three stages of discharge holes Figure 3 Discharge hole area ratio (upper/upper ten lower) B Figure 4 Large flow velocity ratio Figure 5 Figure 6

Claims (1)

[Claims] A submerged nozzle comprising a bottomed cylindrical nozzle with a plurality of discharge ports symmetrically provided in the height direction, the cross-sectional area of the molten steel passage passing through the submerged nozzle in the height direction toward the bottom. to open the discharge holes at the top and bottom of the squeezed part, and the cross-sectional area of each discharge hole (s_1, s_2...s_n in order from the top)
_-_1, s_n) and the cross-sectional area of the molten steel passage corresponding to each discharge hole (S_1, S_2...S_n_-_1
, S_n), the submerged nozzle for continuous casting is characterized by satisfying the following relationship. Note K^2 [S_2/S_1]^3=[S_2+…+S_n
/S_1+S_2+...+S_n]^2...(1) [S_3/S_2]^3=[S_3+...+S_n/S_
2+S_3+...+S_n]^2...(2) [S_n/S_n_-_1]^3=[S_n/S_n_
-_1+S_n]^2...(n) 0.7≦K≦1.0