JPH04238658A - Immersion nozzle for continuous casting - Google Patents

Abstract

PURPOSE:To control molten metal flow and to uniformly flow out discharging flow by surrounding an opening hole in an immersion nozzle with three sides of side walls and bottom wall along long side mold and dividing the opening hole into two with the bottom wall and a parallel middle wall. CONSTITUTION:The opening hole in an immersion nozzle 3 facing to short side mold in the mold for continuous casting at the lower end of a refractory- made hollow guide tube immersed into the molten metal in the mold connected with a discharging hole in the tundish, is surrounded with the three sides with the side walls 2, 2 extended along the long side mold and the bottom wall 1 bridged between the side walls 2, 2. Further, by dividing the opening hole into two with the middle wall 16 arranged in parallel to the bottom wall 1 and extended toward the short side mold from a little inner part of the opening hole and bridged to the side walls 2, 2, the molten metal flow is divided and controlled to uniform flow out the discharging flow.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to a submerged nozzle used for continuous casting of molten metal, especially molten steel, and is suitable for continuous casting to prevent nonmetallic inclusions from being trapped in the solidified shell of a slab. This paper attempts to propose a submerged nozzle for

FIG. 2 shows a general casting situation in a curved continuous casting mold. 5 is molten steel, 6 is an immersion nozzle, 7 is an immersion nozzle discharge port, 8 is a long side mold, 9 is a short side mold, 10
is a solidified shell, 11 is a nonmetallic inclusion, 12 is a bubble, 13
is mold powder.

In such a curved continuous casting machine, molten steel is injected from the tundish through a continuous immersion nozzle pipe via a sliding nozzle or the like, and into a continuous casting mold from a discharge port at the lower end of the immersion nozzle. This mold consists of a long-side mold and a short-side mold, and by cooling from the inside, a solidified shell of the slab is formed, and by continued cooling, this solidified shell is thickened inward and the slab is formed. obtain.

[0004] During such continuous casting, the molten steel discharged from the immersion nozzle into the mold contains nonmetallic inclusions (such as alumina), and these nonmetallic inclusions may accumulate. An inert gas (Ar, N2, etc.) blown in to prevent nozzle clogging due to molten steel is also discharged from the immersion nozzle together with the molten steel. Many of these nonmetallic inclusions and inert gases float toward the molten steel surface in the mold due to the difference in specific gravity with the molten steel, but depending on the flow state of the discharge flow, they may be captured in the solidified slab shell. This will result in a casting defect. This advantage is particularly noticeable when casting is carried out using a curved continuous casting machine. If such nonmetallic inclusions or inert gases are trapped inside the slab, they will cause blisters, which are internal defects, in the subsequent manufacturing process of cold-rolled steel sheets, and they will also be trapped on the surface of the slab or under the skin. In this case, scale and sliver, which are surface defects of the cold-rolled steel sheet, will occur.

[0005] As described above, the capture of nonmetallic inclusions and inert gases that cause internal defects and surface defects in the solidified shell affects the flow condition of the molten steel flow discharged from the immersion nozzle into the continuous casting mold. Therefore, it is important to design the immersion nozzle in such a way that the molten steel flows in a desirable manner.

[0006]

2. Description of the Related Art A typical shape of a conventional immersion nozzle is usually one having two discharge ports at the lower end of the nozzle, as shown in FIG.

However, such a submerged nozzle has the following problems. ■ Due to the influence of the falling energy of the molten steel passing through the immersion nozzle pipe from the tundish, a difference in velocity distribution occurs in the vertical direction above and below the discharge port (see Figure 4).
. In other words, the flow rate of molten steel at the bottom of the discharge port is faster than that at the top of the discharge port, which increases the depth of penetration of bubbles and inclusions into the continuous casting mold, and causes powder entrainment due to increased molten metal level fluctuations. . Such non-uniformity in the flow rate of molten steel cannot be resolved by simply enlarging only the discharge port of the immersion nozzle. (2) Alumina 14 in the steel accumulates in areas where the molten steel flow rate is low at the immersion nozzle discharge port, causing nozzle clogging (see Figure 4). As alumina accumulates and grows, the opening area becomes smaller, which increases the discharge flow rate of molten steel, and the two openings have different areas, which causes uneven flow within the continuous casting mold. This in-mold drift causes an increase in internal defects and surface defects, similar to (2). ■ If the spread of the molten steel flow from the immersion nozzle outlet is large relative to the thickness of the slab, Ar gas and alumina contained in the molten steel flow will be captured at the part where the molten steel flow collides with the solidified shell on the long side. and cause surface defects (see Figure 5).
.

In an attempt to solve the above-mentioned problems of the immersion nozzle, Japanese Patent Laid-Open No. 1-157751 discloses a method in which a plurality of discharge ports are provided in the height direction of a cylindrical nozzle, and the nozzle discharge ports and the nozzle cross-sectional area are set to a predetermined value. A continuous casting immersion nozzle is disclosed in which the flow velocity of molten steel from each discharge port is made uniform by setting the following relationship. However, even if the molten steel flow velocity from the discharge ports is simply made uniform in this way, it is inevitable that a difference in velocity distribution will occur in the longitudinal direction at each discharge port, making it difficult to eliminate the non-uniformity of the molten steel flow velocity. Regarding the spread of the molten steel flow from the nozzle outlet, there remained the problem that Ar gas and alumina contained in the molten steel flow were trapped at the part where the molten steel flow collided with the solidified shell on the long side, similar to the conventional immersion nozzle. .

[0009]

[Problems to be Solved by the Invention] The above-mentioned problems that occur when performing continuous casting using a conventional immersion nozzle are advantageously solved, and non-metallic inclusions and inert gas are trapped in the solidified shell of the slab. It is an object of the present invention to propose an immersion nozzle for continuous casting that can produce slabs with fewer internal defects and surface defects by preventing such occurrence.

0010

[Means for Solving the Problems] The present invention consists of a hollow conduit made of refractory material that is connected to the outlet hole of the tundish and immersed in the molten metal in the continuous casting mold, and has a lower end connected to the continuous casting mold. In an immersion nozzle having an opening facing the short-side mold, the opening is surrounded on three sides by a side wall extending along the long-side mold and a bottom wall extending over the side wall, and arranged parallel to the bottom wall and extending from slightly inside the opening. This immersion nozzle for continuous casting is characterized in that the molten metal flow is divided and controlled by dividing the opening into two by an inner wall extending toward the short side mold and extending over the side walls.

[0011] Controlling the flow of molten steel means suppressing the flow velocity of molten steel from the discharge port, suppressing the flow of molten steel flowing downward from the opening into the continuous casting mold, and controlling the flow of molten steel toward the long-side mold. It is to suppress the mainstream.

FIG. 1 shows the main parts of an example of the immersion nozzle of the present invention. In the figure, 1 is the bottom wall, 2 is the side wall, 3 is the immersion nozzle tube, 4 is the molten steel surface, and 16 is the inner wall. It is surrounded on three sides by an extending side wall 2 and a bottom wall 1 that spans the side wall 2, and is bisected by an inner wall 16 that extends parallel to the bottom wall and extends from slightly inside the opening toward the short side mold. .

[0013]

[Function] First, we will explain how the immersion nozzle of this invention was developed. In view of the shortcomings of conventional immersion nozzles and the mechanism by which defects occur in cast slabs, the inventors conducted various studies and, as a result of consideration, found that the immersion nozzle shown in Fig. 6 was suitable, and filed a patent application earlier. (See the specification of Japanese Patent Application No. 2-293514).

The idea behind developing the shape of such an immersion nozzle was as follows. b) In order to disperse the downward kinetic energy of the molten steel flow passing through the immersion nozzle pipe from the tundish, a bottom wall is provided directly below the immersion nozzle pipe, and the length of the bottom wall in the slab width direction is Directs the flow of molten steel over a certain distance. b) The area of the discharge port of the immersion nozzle is increased to lower the molten steel flow rate so that the molten steel flow uniformly flows into the molten steel in the continuous casting mold. c) To prevent surface defects, prevent the main flow from the immersion nozzle from directly hitting the solidified shell on the long side.

[0015] The immersion nozzle based on this idea is
An opening provided at the lower end of the immersion nozzle is surrounded on three sides by a side wall 2 extending along the long side mold and a bottom wall 1 extending over the side wall 2, and the fluid passing through the immersion nozzle pipe is allowed to flow through the side wall 2.
It flows out at a sufficiently uniform flow rate from the discharge port cross section formed between the bottom wall 1 and the bottom wall 1, and prevents nonmetallic inclusions and inert gas from being trapped in the solidified shell of the slab during actual molten steel casting. be able to.

However, when the pouring rate is low, although the discharge flow uniformly flows out from the opening cross section, the discharge flow velocity and the circulation flow velocity between the nozzle side walls are low, so that inert gas is removed from the molten metal flow. It separates early and floats upwards near the nozzle, causing fluctuations in the melt level near the nozzle (see Fig. 7). Increasing the pouring speed solves the above problem, but if the pouring speed is further increased, the horizontal flow toward the short side mold along the nozzle bottom wall 1 becomes stronger, and the flow from the opening cross section becomes stronger. It was found that the uniformity of the flow rate was disrupted, and the upper part of the discharge port, where the flow rate was low, was likely to become clogged due to the adhesion of alumina-based inclusions (see FIG. 8).

[0017] The inventors have conducted further research on immersed nozzles that can uniformly flow molten steel from the discharge port over a wide range of pouring speeds, and have found that the immersed nozzle according to the present invention is optimal. .

That is, in this invention, the middle wall 16 is arranged parallel to the bottom wall 1 and extends from slightly inside the opening toward the short side mold and extends over the side wall 2, in other words, the middle wall 16 is arranged parallel to the bottom wall 1, and extends between the lower end of the submerged nozzle pipe section 3 and the bottom wall 1. By providing the inner wall 16 with a hole having a diameter smaller than the conduit diameter d0, the discharge flow is divided into upper and lower parts, and two circulating flows are formed in the longitudinal direction. The flow velocity of each circulating flow is approximately equal, and approximately twice the circulating flow velocity shown in FIG. Therefore, early separation of the inert gas from the molten metal flow is eliminated, and concentrated upward levitation of the inert gas is suppressed.
Fluctuations in the hot water level became extremely small (see Figure 9). Furthermore, over a wide range of pouring speeds, there were no areas where the flow rate was slow throughout the discharge flow (see FIG. 9), and there were no areas where alumina-based inclusions were concentrated and adhered.

By the way, in such a submerged nozzle, the length a in the vertical direction of the opening of the submerged nozzle (indicated by the same reference numeral in FIG.
It is desirable to determine the (inner diameter of the submerged nozzle tube).

Therefore, using the vertical length a of the opening of the immersion nozzle, the bottom wall length b, and the side wall spacing d0 as parameters, the penetration depth of the bubbles that cause blistering into the continuous casting mold is calculated using a water model. We investigated. the result,
In order to reduce the bubble penetration depth and prevent blistering defects, a≧100 mm, b≧40 mm, d0
≧50mm
…■ was optimal.

[0021] The vertical length a of the opening of the immersion nozzle is 1
If the diameter is less than 0.0 mm, the velocity of the fluid flowing out from the outlet of the submerged nozzle becomes faster in the direction of the lower part of the outlet, and the flow impinges on the short-side mold and then generates a fast flow that flows down the short side. The bubbles penetrate deeply into the continuous casting mold along this downward flow on the short side.

[0022] When the bottom wall length b is less than 40 mm, the bottom wall cannot suppress the downward spread within the continuous casting mold of the flow falling in the immersion nozzle pipe, and some of the flow flows downward into the continuous casting mold. invade. In addition, sufficient circulating flow cannot be formed, resulting in velocity distribution from the discharge flow, and the penetration depth of bubbles increases.

[0023] When the side wall spacing d0 is less than 50 mm,
Since the cross-sectional area at the immersion nozzle discharge port becomes smaller, the discharge flow velocity particularly from the lower side of the cross-section becomes faster, the downward flow velocity along the short side becomes faster, and the bubble penetration depth becomes larger.

Furthermore, as a result of repeated experiments with actual equipment for the submerged nozzle under the condition (2), it has been found that it is preferable that the optimum value of the submerged nozzle shape satisfies the following equations.

First, the distance between the immersion nozzle and the continuous casting mold at the meniscus is (x/2)-{(d0 +2t)/2}≧20
(mm) It is preferable that it is ■. Here, x is the slab thickness (mm), and t is the thickness of the side wall 2 (
mm).

[0026] If the above-mentioned condition (2) is not met, the solidified shell that has grown from the long-side mold side will come into contact with the immersion nozzle, increasing the probability that the nozzle will break. t needs to be 10 mm or more from the viewpoint of refractory strength.

Next, regarding the maximum value in the width direction of the slab at the bottom of the immersion nozzle, b+(d0/2)≦270 (mm)
…■
It is preferable that Here, d0 is the length (mm) of the submerged nozzle pipe portion in the slab width direction. When the width of the immersion nozzle became too wide, the water flow between the nozzle in the nozzle body and the continuous casting mold became poor, and alumina was easily trapped due to insufficient cleaning effect on the long side shell.

Furthermore, the immersion depth of the immersion nozzle in the bath is c≦500 (mm).

...It is preferable that it is ■. When the condition of formula (1) above was not met, there was a high probability that the nozzle would be damaged due to contact with the long side shell at the lower end of the submerged nozzle. In addition, the distance (ca) between the meniscus and the upper end of the immersion nozzle opening should be ca≧60 mm in order to prevent powder from being drawn in from the meniscus near the nozzle.

...It is preferable that it is ■.

Furthermore, the width of the discharge port of the immersion nozzle in the thickness direction of the slab, ie, the side wall spacing d0, is related to the slab thickness x, as follows: 0.2≦d0/x≦0.7

...It is preferable that it falls within the conditions of ■. d0 /x＞
If it is 0.7, air bubbles will be trapped in the solidified shell on the long side, increasing surface defects, while if d0/x<0.2, the fluid level will fluctuate violently, probably because the mainstream collides with the mold on the short side. ,
Powdery surface defects increase.

Furthermore, in this invention, the pore diameter d of the inner wall is
, the height g of the inner wall from the bottom wall and the distance f from the end of the inner wall to the side wall are expressed by the following formulas: 3d0 /4≦d≦d0

…■ a/2≦g≦3a/4

…■ b/6≦f≦b

...It is preferable to satisfy ■.

If the above conditions (1) and (2) are not met, the upper and lower divisions of the discharge flow due to the inner wall will be biased, and the discharge flow with a higher discharge flow rate will not form a circulation flow but a strong flow in the horizontal direction. , which increases the downward flow velocity along the short sides of the mold and increases the bubble penetration depth. Furthermore, even if the condition (2) above is not met, the flow becomes non-uniform. For example, when f<b/6, the inner wall obstructs the upward flow in the lower discharge flow, and the horizontal flow becomes stronger. When f>b, a part of the upper discharge flow flows downward because the width of the inner wall is narrow, and no circulating flow is generated.

[0032]

[Example] Under the casting conditions shown in Table 1, ultra-low carbon Al-killed steel was cast using a continuous casting machine with a bending radius of 12 m.

[0033]

Seven types of nozzles shown in Table 2 were used as the immersion nozzles.

[0035]

As the casting heat, 12 heats (250
t/heat) was cast with each nozzle. Table 3 compares the blistering index of cold-rolled steel sheets for each type of nozzle used.
Shown below. The blister occurrence index is: defective coil length/total coil length = 0
．． 05 was defined as index 1.

[0037]

From the same table, the use of the immersion nozzle according to the present invention resulted in a flow situation in the mold similar to the result of the water model, and the blistering was drastically reduced.

Next, in order to investigate the influence of the presence or absence of an inner wall on the level fluctuation by varying the pouring speed, Table 2 was used.
No. in Using the immersed nozzle of No. 2 (invention) and an immersed nozzle with the same size except for the inner wall, the amount of fluctuation in the melt level at a position 50 mm away from the nozzle periphery in the direction of the short side mold was investigated. The results are shown graphically in FIG. Note that the same table shows that the pouring rate of the immersion nozzle of the present invention is 3.0 t/
Relative evaluation is shown with the amount of fluctuation in the hot water level at min as 1. As is clear from the figure, the immersion nozzle of the present invention suppresses fluctuations in the molten metal level even when the pouring speed becomes low.

Next, the effect of the presence or absence of an inner wall on the thickness of deposits on the nozzle was investigated by varying the pouring speed. As above, the immersion nozzle is No. 2 in Table 2. 2 (present invention)
The deposit thickness at the measurement position shown in FIG. 11 is graphically shown in FIG. 12 as a relative evaluation with respect to d0 using a submerged nozzle with the same size as the one without the inner wall. As is clear from the same table, the immersion nozzle of the present invention has small variations in deposit thickness.

Next, the pouring speed was varied to examine the effect of the presence or absence of an inner wall on the frequency of blistering in cold rolled steel sheets. As above, the immersion nozzle is No. 2 in Table 2. 2
Using the immersed nozzle of the present invention and a immersed nozzle with the same size except for the inner wall, we calculated the frequency of blistering at a pouring rate of 2.4 t/min in the immersed nozzle without an inner wall. A relative evaluation of 1 is shown in a graph in FIG. As is clear from the same table, the immersion nozzle of the present invention has a stable blistering frequency at a low level over a wide range of pouring speeds.

[0042]

Effects of the Invention The continuous casting immersion nozzle of the present invention has an opening facing the short side mold of the continuous casting mold surrounded on three sides by a side wall extending along the long side mold and a bottom wall extending over the side wall. By dividing the opening into two by an inner wall arranged parallel to the opening and extending from slightly inside the opening toward the short side mold, the flow of the molten metal flowing out through the opening can be controlled and a wide range of pouring speeds can be achieved. Over the course of
The discharge flow can flow uniformly from the discharge port cross section,
It is also possible to prevent concentrated floating of inert gas at low pouring speeds, and to drastically reduce nozzle clogging.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an example of a continuous casting immersion nozzle according to the present invention.

FIG. 2 is a diagram showing a general casting situation in a continuous casting mold.

FIG. 3 is a diagram showing the main parts of a conventional submerged nozzle.

FIG. 4 is a diagram showing the flow velocity distribution of molten steel at the discharge port of a conventional immersion nozzle.

FIG. 5 is a diagram showing how a molten steel flow spreads into a continuous casting mold when a conventional immersion nozzle is used.

FIG. 6 is an explanatory diagram of a submerged nozzle previously developed by the inventors.

FIG. 7 is a diagram showing the flow situation in the mold when the immersion nozzle developed earlier by the inventors is used when the pouring rate is low.

FIG. 8 is a diagram showing the flow situation in the mold when the immersion nozzle developed earlier by the inventors is used when the pouring rate is high.

FIG. 9 is a diagram illustrating an example of the immersion nozzle for continuous casting of the present invention and the flow state of fluid within the nozzle when the nozzle is used.

FIG. 10 is a graph showing the relationship between the amount of fluctuation in the melt level and the pouring speed in the vicinity of the immersion nozzle.

FIG. 11 is an explanatory diagram showing the measurement position of the thickness of deposits on the immersion nozzle.

FIG. 12 is a graph showing the relationship between the thickness of deposits on the immersion nozzle and the pouring rate.

FIG. 13 is a graph showing the relationship between the frequency of blistering in a cold rolled steel sheet and the pouring rate.

[Explanation of symbols]

1 Bottom wall 2 Side wall 3 Immersion nozzle pipe section 16 Middle wall

Claims (1)

[Claims] Claim 1: Consisting of a hollow conduit made of refractory that is connected to the outlet hole of the tundish and immersed in the molten metal in the continuous casting mold, and has an opening at the lower end toward the short side mold of the continuous casting mold. In the immersion nozzle, the opening is surrounded on three sides by a side wall extending along the long side mold, and a bottom wall spanning the side wall, and a side wall arranged parallel to the bottom wall and extending from slightly inside the opening toward the short side mold. 1. A submerged nozzle for continuous casting, characterized in that a molten metal flow is divided and controlled by dividing the opening into two by a middle wall spanning the length of the opening.