JPH04344854A - Tundish for continuous casting - Google Patents

Abstract

PURPOSE:To provide tundish shape by which surface defect and inner defect caused by slag and non-metallic inclusion flowing in the tundish from a ladle are not developed in successive part between the previous heat and the following heat in continuous casting. CONSTITUTION:In the tundish for continuous casting providing a recessed part, at the time of starting the pouring from a following ladle, the vol. of the recessed part in the tundish formed in a molten steel pouring flow dropping range is decided so as to be slag involving index (epsilon/H<2>)<1/33 indicated with a function using EPSILON to be the ratio of the max. kinetic energy of the molten steel poured through a long nozzle and the molten steel quantity existing in stirring range and H for the molten steel depth existing in the stirring range just before pouring from the following ladle to be to the min. molten steel depth. In the stirring range in the tundish, the involution of slag is prevented and in the float-up area, the floating-up property of the slag is improved and the good quality cast slab is obtd. without product defect at the successive part between the previous heat and the following heat.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a tundish shape that does not cause surface defects or internal defects in products due to slag and nonmetallic inclusions flowing into the tundish from the ladle in the field of continuous casting. .

0002

[Prior Art] In continuous casting operations, when the same tundish is used to continuously cast multiple charges, slag and non-metallic inclusions flow into the mold at the joints between the charges, resulting in quality deterioration. is generally well known.

At the joint, after pouring molten steel into the tundish of the front ladle, the ladle is replaced and the next ladle is poured with molten steel. It takes several minutes to replace the ladle, and during that time the weight of the molten steel in the tundish decreases, and the depth of the molten steel also becomes shallow.

[0004] When the ladle is next opened, the initial injection amount from the ladle must be larger than the casting amount due to operational reasons such as fully opening the ladle sliding nozzle and recovering the amount of molten steel in the tundish.

For this reason, the slag floating on the tundish is re-engulfed by the reverse flow of the initial large injection flow, and the molten steel re-oxidizes the engulfed slag, increasing the concentration of non-metallic inclusions and causing slag to flow into the mold. There is an influx.

[0006] One of the causes of product defects at joints is the above-mentioned slag entrainment. Conventional measures to prevent deterioration of slab quality include (1) preventing slag from flowing out from the ladle to the tundish, and (2) ensuring slag flotation in the tundish.

Regarding (1) slag flowing out from the ladle into the tundish, various countermeasures have been taken in the past, but it has not been possible to completely eliminate the slag flowing out, and the problem of re-engulfment of the spilled slag has not been solved. I haven't.

(2) Ensuring the slag floating property of the tundish is handled by using a large capacity tundish, but the slag floating property when the ladle injection amount and casting amount are equal (steady operation) is discussed. However, it is not designed to deal with the initial large injection flow at the joint, nor is it a measure to prevent slag entrainment.

[0009] As described above, it can be said that conventionally, there was no technique for dealing with an increase in the initial injection amount when replacing the ladle, such as re-engaging of the slag floating on the tundish or flowing into the mold.

[0010] Japanese Patent Publication No. 52-36085 discloses a tundish provided with a concave portion serving as a stirring tank for the purpose of removing non-metallic inclusions due to the addition of powder additives.
Japanese Unexamined Patent Publication No. 62-64461 discloses a tundish having a recessed portion to improve the floating property of non-metallic inclusions when the ladle injection amount and casting amount are equal (regular operation). In neither case is there any discussion of prevention of slag entrainment, and there is no design concept for solving the present invention, nor is there any suggestion.

[0011]

[Problems to be Solved by the Invention] In order to eliminate product defects at the joint between charges, the present invention aims to prevent slag from being caught in the tundish, which causes such defects, and improves slag flotation in the tundish. It is something.

0012

[Means for Solving the Problems] The gist of the present invention is as follows.

[0013] In a tundish for continuous casting, a recess is formed in the falling area of the molten steel injected from the ladle through a long nozzle, and a floating area for slag and nonmetallic inclusions is provided downstream of the falling area of the injection stream. At the start of the next pot injection, the maximum kinetic energy (1
/2・ρ・Q・v2) and the amount of molten steel (ρ・S・L) existing in the molten steel injection flow falling area (inside the stirring area) of the tundish with a recessed part is expressed by the following equation 1. Let ε,
Slag entrainment index (ε/H2) 1 expressed as a function of the molten steel depth H existing in the molten steel injection flow fall area (inside the stirring area) of the tundish that forms the concave part immediately before the next pot injection where the molten steel depth is the minimum The depth of the recess of the tundish formed in the molten steel injection flow fall area such that /3 is smaller than 3,
This is a tundish for continuous casting, characterized in that the length and width (recess volume) are determined.

[0014]

[Mathematical 1] ε=[(1/2・ρ・Q・v
2)/(ρ・S・L)]x1000 However, ε:
Stirring energy (W/t) ρ: Molten steel density (ton/m3
) Q: Molten steel injection amount (long nozzle discharge flow rate) (m3/s
) v: Long nozzle discharge flow rate (m/s) S: Stirring area width direction cross-sectional area (m2) L: Stirring area length (m) H: Stirring area molten steel depth (m)

[0015] Furthermore, in the above-mentioned tundish, the tundish for continuous casting is such that the side surface of the recess on the floating area side that separates the molten steel injection flow falling area from the floating area is inclined toward the floating area.

[0016]

[Operation] In order to solve the above problem, we conducted a water model test that simulated the initial stage of pouring into the next ladle when replacing the ladle, and clarified the slag entrainment behavior. The water model experimental equipment is shown in Figure 11.
The model shown in was used. 1 is a ladle and 2 is a long nozzle.

In order to quantify the amount of slag entrainment, a weir 4 as shown in FIG. The ratio of the weight of the passing tracer to the weight of the tracer floating in the stirring area was regarded as the slag entrainment rate.

An experiment was conducted by keeping the amount of liquid flowing into the mold from the immersion nozzle 6 constant, and varying the conditions of the amount Q of liquid poured from the ladle 1 and the depth H of the liquid in the tundish 7.

From the water model test, the slag-equivalent tracer 5 is caught up in the reverse flow 8 of the ladle injection flow.
The larger the reverse flow 8 is, the larger the entrainment rate of the slag-equivalent tracer 5 is. The reverse flow increases in proportion to the stirring energy ε of the ladle injection flow, and attenuates in proportion to the square of the liquid depth H in the injection flow falling area. The slag entrainment phenomenon is thought to be caused by the flow velocity at the slag-metal interface due to reverse flow, and entrainment does not occur unless the velocity exceeds a certain critical velocity.

From the water model results shown in FIG. 2, the entrainment of the slag floating on the tundish is determined by (ε/H2)1/3, regardless of the injection amount Q from the ladle and the liquid depth H. The larger (ε/H2)1/3 is, the larger the entrainment rate of the slag-equivalent tracer 5 is. Also, (ε/H2
)1/3 is smaller than the critical value 3, no slag entrainment occurs. That is, it can be said that (ε/H2)1/3 represents the flow velocity at the slag-metal interface due to reverse flow.

From the above, the function (ε/H2)1/3 found in the water model is considered to be a general expression for the slag entrainment behavior, and this function is regarded as the slag entrainment index. In order to prevent slag from reoxidizing molten steel and preventing it from flowing into the mold, it is necessary to eliminate slag entrainment, and the slag entrainment index must be made smaller than the critical value shown in FIG. 2.

A specific method is to lower ε and increase the depth of molten steel. In order to reduce ε, it is possible to reduce the initial injection amount when replacing the ladle, but it is difficult to suppress the injection amount due to operational reasons.On the other hand, there is no way to reduce ε by increasing the overall capacity of the tundish. , hard constraints, and high costs are concerns.

The most effective method for reducing the slag entrainment index is to increase the depth of the molten steel.In order to reduce the amount of entrainment in a compact and low-cost manner, only the falling area of the injection flow is concave, and the depth of the molten steel in the stirring area is reduced. It is to raise the level. For example, as shown in FIG. 1, slag entrainment can be prevented by making the stirring region concave and designing the depth of the concave portion and the amount of molten steel to be smaller than the critical slag entrainment index 3 to prevent slag entrainment.

Although good results were obtained in preventing slag entrainment in the tundish in which the falling area of the injected flow was made concave and the concave portion was designed to be smaller than the critical slag entrainment index of 3, furthermore, the slag that flows from the ladle into the tundish was obtained. We investigated the problem that slag and nonmetallic inclusions do not float to the tundish and flow directly into the mold due to the direct flow caused by the large injection flow at the joint.

A water model experiment was conducted in which a tracer equivalent to slag was added from the ladle, and the behavior of slag flowing from the ladle into the mold was clarified. The ratio of the weight of the tracer equivalent to slag flowing into the mold and the weight of the tracer added to ladle 1 was regarded as the ladle slag mold flow rate.

In the water model experimental apparatus shown in FIG. 3, the injection amount Q from the ladle 1, the liquid depth H, the depth h of the recess 10, and the amount flowing into the mold from the immersion nozzle 6 are constant,
Experiments were conducted by varying the inclination θ of the side surface on the floating region side of the recess 10 that separates the ladle injection flow fall region 3 and the floating region 9.

The water model test shows that when the inclination θ becomes large, a direct flow is generated in the tundish, and the flotability of the flotation area 9 deteriorates. Further, when the inclination θ is small, direct flow within the tundish is prevented, but the upward flow from the recessed portion to the floating region becomes strong, a vortex is generated, and the floating performance of the floating region 9 is reduced.

From the results of the water model test shown in FIG. 4, the buoyancy of the slag is optimal when the inclination θ of the side surface of the recess 10 on the floating region side is 10° to 80°. By designing with this optimal inclination, it is possible to realize an upward flow of molten steel without generating a vortex in the floating area 9, to prevent a direct flow of molten steel from flowing directly into the mold from the long nozzle 2, and to prevent slag and non-metallic metal from flowing from the ladle. It is possible to prevent inclusions from flowing into the mold.

[0029]

Embodiments Examples of the present invention will be described below with reference to the drawings.

The present invention was applied to a tinplate plate suitable for DI cans. First, a normal tundish without a concave portion prior to implementation of the present invention was prepared. Its specifications are shown below. Molten steel capacity in tundish: 15t Tundish total length: 7.4m

FIG. 12 shows the operating conditions of the joint portion between the charges in a normal tundish having no recesses, which is the object of the present invention. The initial injection amount when replacing the ladle is
The molten steel depth H immediately before pouring into the next pot is 350 mm, and the molten steel capacity in the tundish is 10 tons.

Next, Example 1 will be explained.

In order to prevent slag from reoxidizing the molten steel and preventing it from flowing into the mold, a recess is formed in the molten steel injection flow fall area, and the depth and volume of the recess are designed to prevent slag from being entrained.

From the above casting conditions for the tin plate for DI cans,
The slag entrainment index before implementation of the present invention was about 5,
In a normal tundish that does not have a recessed part, slag entrainment occurs at the joint between the charges, causing quality deterioration.

Therefore, a recess is formed in the molten steel injection flow falling area, and the molten steel injection flow falling area is set so that the slag entrainment index (ε/H2) 1/3 < 3 is satisfied when the molten steel depth is the depth immediately before the next injection into the ladle. design.

The length L of the molten steel injection flow falling area (within the stirring area) of the tundish in which the concave portion is formed is equal to the initial injection amount 5.
Take the length of the stirring influence zone corresponding to ton/min, and
．． It is determined to be 7m.

As shown in FIG. 1, the taper of the tundish was the same as before the present invention, and a recess 10 was attached to the bottom of the tundish. The molten steel depth was set to the depth immediately before the next pot injection. At this time, the slag and
The depth of the floating region of nonmetallic inclusions was 350 mm.

The molten steel conditions immediately before pouring into the next ladle had the following values. Molten steel density ρ: 7.2 (ton/m3) Molten steel injection amount (long nozzle discharge flow rate) Q: 0.0116 (m3/s) Long nozzle discharge flow velocity v: 1.23 (m/s) Floating area width direction cross section Area S': 0.1873 (m2) Stirring area length L: 2.7 (m)

The amount of molten steel (ρ・S・
The dimensions are determined so that the relationship between L) and H satisfies the above condition of slag entrainment index <3.

[0040]ρ, Q, v are given as conditions,
The kinetic energy of molten steel 1/2・ρ・Q・v2 was calculated. (ε/H2)1/3 <3, formula (1): (1
/2・ρ・Q・v2)x(1000/ρ・S・L)x
(1/H2)<27 was obtained. From FIG. 1, the widthwise cross-sectional area of the stirring region S=S'+0.35xh=0.1873+0.
35xh and stirring zone molten steel depth H=0.35+h were obtained. Therefore, equation (1) is a function only of the recess depth h, and h
>0.295 (m) was obtained. Therefore, h was determined to be 300 mm, and S, H, and ε were calculated.

The determined dimensions are shown below. Stirring zone width direction cross-sectional area S: 0.2923 (m2) Stirring zone molten steel amount ρ・S・L: 5.68 (ton) Stirring energy ε: 11.12 (W/t) Stirring zone molten steel depth H: 6
50mm

Therefore, the depth h of the recess was determined to be 300 mm.

[0043] By using the design method described above, it was possible to design a tundish that does not involve slag. This design method is
It is also effective for high ton/min, as shown in Figure 5.
It can also be applied to ladle 2 strands.

[0044] Furthermore, if there is a hard constraint and the depth of the recess is limited, the width of the tundish portion in which the recess is formed is made wider than the other tundish portions to reduce the required depth of the recess, as shown in Fig. 6. I can deal with it.

Next, Example 2 will be explained.

In order to prevent slag and nonmetallic inclusions flowing from the ladle from flowing into the mold in a tundish in which the falling region of the injection flow is concave and the concave portion is designed to be smaller than the critical slag entrainment index 3, the method shown in FIG. The inclination of the side surface of the recess 10 on the floating region side is determined so that the slope of the side surface of the recess 10 on the floating region side is 10° to 80°.

[0047] An actual tundish was manufactured in which the falling area of the injection flow was concave, the concave part was designed to be smaller than the critical slag entrainment index of 3, and the side surface of the concave part on the floating area side had an inclination of 60 degrees. A casting experiment was conducted using an actual machine for oriented tin plate.

FIG. 8 shows the slag entrainment index (ε/H2) 1/3 when the modified tundish of the present invention is used in an actual machine. Immediately after the start of the next ladle injection, the slag entrainment index increases rapidly, but it stabilizes at a lower level than when the amount of injection from the ladle and the amount of casting become the same. Although this tendency is the same for ordinary tundishes that do not have recesses, the slag entrainment index of the tundish of the present invention is significantly lower than that of ordinary tundishes that do not have recesses.

FIG. 9 shows the results of evaluation using the magnetic particle flaw detection method as a quality index, and the correspondence with the slag entrainment index. In the figure, the ● mark indicates a product using the tundish of the present invention, and the ○ mark indicates a product using a normal tundish having no recesses. As can be seen from FIG. 9, when the slag entrainment index is less than 3, the rate of occurrence of magnetic particle detection defects is stable at a low level, and as shown in FIG. 10, there are few cracks that occur during the manufacturing of DI cans and the like.

[0050] As described above, the quality of the tundish of the present invention and the normal tundish without recesses can be well classified by the slag entrainment index, and the tundish of the present invention eliminates surface defects and internal defects of the tin plate even at joints. It was confirmed that slabs with no occurrence of .

[0051]

[Effects of the Invention] The present invention constructed as described above has the following effects.

[0052] To prevent the slag floating on the tundish at the joint between the charges from being re-engulfed by the reverse flow of the initial large injection flow, and to eliminate product defects at the joint between the charges due to slag entrainment. I can do it.

[0053] By realizing an upward flow of molten steel without generating a vortex in the floating area of the tundish, and by preventing a direct flow from flowing directly into the mold from the long nozzle of the tundish, slag and non-metallic materials flowing from the ladle are prevented. It is possible to prevent inclusions from flowing into the mold.

[0054] It is possible to prevent the slag that has flowed from the ladle into the tundish from flowing downstream at the joint between the charges, and it is possible to produce highly clean slabs with no surface defects or internal defects at the joint. It is possible.

[Brief explanation of the drawing]

FIG. 1(a) is an explanatory longitudinal cross-sectional view showing a structural example of a tundish for continuous casting according to an embodiment of the present invention immediately before pouring the next ladle at which the molten steel reaches the minimum depth. Figure 1(b) is Figure 1
It is an AA vertical cross-sectional view of (a).

FIG. 2 is a graph showing the relationship between the tundish slag entrainment rate and the slag entrainment index (ε/H2) 1/3.

FIG. 3 is an explanatory longitudinal cross-sectional view showing the structure of the water model experiment apparatus of the present invention.

FIG. 4 is a graph showing the relationship between the inflow rate of ladle slag into the mold and the inclination of the side surface on the floating region side.

FIG. 5 is an explanatory longitudinal cross-sectional view showing a structural example of a tundish for continuous casting according to an embodiment of the present invention.

FIG. 6(a) is an explanatory longitudinal cross-sectional view showing a structural example of a tundish for continuous casting according to an embodiment of the present invention. Figure 6 (
b) is an explanatory plan view of FIG. 6(a).

FIG. 7(a) is an explanatory longitudinal cross-sectional view showing a structural example of a tundish for continuous casting according to an embodiment of the present invention immediately before the next ladle injection when the molten steel reaches the minimum depth. Figure 7(b) is Figure 7
It is an AA vertical cross-sectional view of (a).

FIG. 8 is a graph showing the relationship between the slag entrainment index (ε/H2) 1/3 and the elapsed time from the opening of the ladle.

[Figure 9] Number of magnetic particle detection defects and slag entrainment index (ε/
It is a graph which shows the relationship with H2)1/3.

FIG. 10 is a graph showing the relationship between the DI can cracking occurrence rate and the number of defects detected by magnetic particle testing.

FIG. 11 is an explanatory longitudinal cross-sectional view showing a structural example of a conventional continuous casting tundish.

FIG. 12 is a graph showing changes over time in the molten steel capacity in a conventional tundish without a concave portion and the initial injection amount when replacing the ladle.

[Explanation of symbols]

1 Ladle 2. Long nozzle 3 Ladle pouring fall area (stirring area) 4 Weir 5 Slag equivalent tracer 6 Immersion nozzle 7 Tundish 8 Reverse flow 9 Levitation area 10 Recess

Claims (2)

[Claims] 1. A tundish for continuous casting in which a recess is formed in the falling area of the molten steel injected from the ladle through a long nozzle, and a floating area for slag and nonmetallic inclusions is provided downstream of the falling area of the injection stream. At the start of injection into the next ladle, the maximum kinetic energy (1/2・ρ・Q・v2) of the molten steel injected through the long nozzle and the falling area of the molten steel injection flow in the tundish (within the stirring area) that formed the concave part. Let ε be the ratio to the amount of molten steel (ρ・S・L) present in (within stirring area)
The depth and length of the concave part of the tundish formed in the molten steel injection flow fall area so that the slag entrainment index (ε/H2) 1/3 expressed as a function of the molten steel depth H existing in the area is smaller than 3. A tundish for continuous casting, characterized in that the width (recess volume) is determined. [Mathematical 1] ε=[(1/2・ρ・Q・v
2)/(ρ・S・L)]x1000 However, ε:
Stirring energy (W/t) ρ: Molten steel density (ton/m3
) Q: Molten steel injection amount (long nozzle discharge flow rate) (m3/s
) v: Long nozzle discharge flow rate (m/s) S: Stirring area width direction cross-sectional area (m2) L: Stirring area length (m) H: Stirring area molten steel depth (m) 2. The continuous casting tundish according to claim 1, wherein a side surface of the recess on the floating area side of the recess separating the molten steel injection flow falling area and the floating area is inclined toward the floating area.