JP2006239746A - Tundish for continuous casting of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tundish to be used for the continuous casting of steel which effectively eliminates alumina-based inclusions so as to enhance the cleanliness of a cast steel billet by lessening the alumina-based inclusions in the cast steel billet. <P>SOLUTION: The invented tundish 3 for the continuous casting of steel has a plurality of weirs 9 and 10 between an inlet 2 for the molten steel from a ladle and an outlet 4 for the molten steel to a casting mold. The weir 9 of both adjoining weirs 9 and 10 has an opening 9a which enables the molten steel to flow only in the direction of either side wall of the upper portion of a molten steel bath. The weir 10 has an opening 10a in the direction of the side wall opposite to the opening 9a, of the lower portion of the molten steel bath. The width (w) and height (h) of each of the openings 9a and 10a of the weirs 9 and 10, respectively meet adequate conditional expressions. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a tundish for producing a cast slab having a high degree of cleanness by efficiently levitating alumina inclusions in molten steel, which is used when continuously casting steel to produce a slab.

  Low carbon steel sheets are used in various processing applications, and especially Ti-containing ultra-low carbon steels are processed into complex shapes such as automotive outer panels. At this time, if alumina inclusions remain in the slab, defects such as surface flaws and cracks during processing occur. Molten steel is melted by decarburization and component adjustment in a converter, but aluminum, which is a strong oxide-generating element, must be added for deoxidation to remove dissolved oxygen in the molten steel after decarburization. In general, alumina is formed at that time.

  Alumina floats due to the specific gravity and is discharged from the molten steel, but not all floats, but it is brought from the ladle through the tundish through the immersion nozzle into the mold and remains in the cast slab. Some will do. This causes the above defects. In addition, it adheres to the inner wall of the immersion nozzle that injects molten steel into the mold, causing nozzle clogging, causing drift in the mold and unstable operation. In order to reduce the harmfulness of such alumina inclusions, various methods for reducing alumina inclusions in molten steel have been proposed.

  As one of them, a method for controlling the flow of molten steel in the tundish and promoting the floating removal of alumina inclusions has been proposed. As an example, Patent Document 1 (Japanese Patent Laid-Open No. 7-132353) will be described with reference to FIG. In the tundish 3, there are three weirs (lower weir 6, upper weir 7, lower weir 8) between the inlet 2 and the outlet 4 from the ladle 1 to promote the floating of inclusions. ) And the conditions for the height and spacing of each weir are defined by mathematical formulas. Each of the weirs 6, 7, 8 extends over the entire width of the tundish 3, and the upper part of the lower weir 6 and the lower weir 8 and the lower part of the upper weir 7 serve as a flow path for molten steel. Below the outlet 4 is provided an immersion nozzle 5 for injecting molten steel into the mold.

The technical point of this patent document 1 is to form an upward flow that causes the inclusions to float up to the surface of the molten steel, and to optimize the upward flow velocity so that inclusions and slag that floats near the surface are not involved. . For this purpose, the conditions of the height and spacing of each weir 6, 7, 8 are defined by mathematical formulas. On the other hand, for example, when the lower weir 6 is too low, a short-circuit flow is formed and an upward flow is not formed, and the inclusion floating effect is reduced. On the other hand, when the lower weir 6 is too high, or when the interval between the lower weir 8 adjacent to the upper weir 7 is too narrow, the upward flow becomes excessive, which disturbs the molten steel surface and involves inclusions and slag. In this way, the lower weir and upper weir can be arranged to form an upward flow in the tundish. However, in recent years, it has not always been able to meet the increasingly severe need for inclusion removal, and further inclusion reduction measures are not available. It has been demanded.
JP-A-7-132353

  Three mechanisms can be considered to promote the floating removal of inclusions in the tundish. That is, (1) to form an upward flow to the molten metal surface, (2) to increase the residence time in the tundish and hence the floating time of inclusions, and (3) to promote the aggregation and coarsening of inclusions. As described in Patent Document 1 above, the method of arranging the lower weir and the upper weir over the entire width controls the flow in the tundish in the vertical direction. Therefore, (1) Upflow to the hot water surface Its main purpose is formation. The other effects of (2) increase in residence time in tundish and (3) acceleration of aggregation coarsening are hardly considered. In order to further reduce inclusions, it is important to utilize the mechanisms (2) and (3) in addition to the mechanism (1).

  An object of this invention is to provide the tundish which improved the floating removal function of the alumina type inclusion.

The inventors of the present invention have made extensive studies through flow calculation, actual slab investigation, etc., and have completed the present invention in order to enhance the floating removal function of alumina inclusions in the tundish. That is, according to the present invention, the tundish in the continuous casting of steel has a plurality of weirs between the injection portion from the ladle and the outlet to the mold, and at least of the plurality of weirs One of the two adjacent weirs has an opening through which molten steel can circulate only in the side wall direction on the upper side in the height direction of the molten steel bath, and the other weir has the opening in the lower direction in the height direction of the molten steel bath. A tundish for continuous casting is provided, which has an opening only in the direction opposite to the side wall and each weir satisfies the following formulas (1) and (2).
1/6 ≦ w / W ≦ 2/5 (1)
− (5/4) · (w / W) + 17/24 ≦ h / H ≦ 3/4 (2)
here,
W: The width of the tundish molten steel surface,
H: Depth of tundish molten steel surface,
w: width of the opening of the portion of the weir immersed in the molten steel,
h: Height of the opening of the portion of the weir immersed in the molten steel

  In this continuous casting tundish, a weir with an opening through which molten steel can flow only in the direction of the side wall on the upper side of the molten steel bath is arranged on the side close to the molten steel injection section from the ladle. A weir having an opening only in the side wall direction opposite to the opening in the lower part of the height direction may be arranged on the side close to the outlet.

  According to the present invention, since the alumina inclusions are removed with high efficiency in the tundish, it is possible to manufacture a slab having a high cleanliness. The steel plate produced from this slab has excellent quality with few surface defects caused by alumina inclusions and cracks during processing.

  Next, the best mode for achieving the effects of the present invention will be described with reference to the drawings. In the present specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 2 is a top view and a side view of a continuous casting tundish 3 (hereinafter referred to as “tundish 3”) according to an embodiment of the present invention. A plurality of weirs are provided in the tundish 3 between the inlet 2 from the ladle and the outlet 4 to the mold. In the embodiment shown in FIG. 2, as an example, a mode in which two weirs 9 and 10 are provided in the tundish 3 is shown. At this time, the first weir 9 blocks the flow in the lower part of the molten steel bath in the tundish 3 and has an opening 9a through which the molten steel flows, and the second weir 10 is in the tundish 3. The flow of the upper part of the molten steel bath in the height direction is blocked, and an opening 10a through which the molten steel flows is provided at the lower part. In FIG. 2, the first weir 9 having an opening 9 a at the top is arranged on the side close to the inlet 2.

  The openings 9a and 10a of the first and second weirs 9 and 10 are not part of the full width of the tundish 3 molten steel surface, but a part thereof. First, in the first weir 9, an opening 9a is provided in the upper part and is located on one side of the side wall, and the flow of molten steel on the other side is blocked. Moreover, in the 2nd dam 10, the opening part 10a is provided in the lower part, is located on the opposite side to the opening part 9a of the 1st dam 9, and the distribution | circulation of the molten steel of the other side is interrupted | blocked. By installing the weirs 9 and 10 having such openings 9a and 10a, the length of the molten steel flow path between the adjacent weirs 9 and 10 is increased, thereby increasing the stay time of the molten steel. It was thought that the floating of inclusions caught in

  However, not only simply providing the openings 9a and 10a, it is considered that there are suitable conditions for the width w and height h of the openings 9a and 10a of the first and second weirs 9 and 10, Analyzing the flow in the tundish 3 under various conditions of the width w and height h of 10a, introducing a predetermined amount of inclusions from the inlet 2 and arranging a predetermined amount of slag on the molten metal surface Thus, the distribution trajectory of the inclusion was traced, and a simulation was carried out to see how the ratio of flowing out from the outlet 4 at the bottom of the tundish 3 changes.

  The analysis conditions were such that the total length of the tundish 3 was 8 m, the molten steel amount was 60 t, and the injection amount was 10 t / min. The results are shown in FIG. 3, and the widths w and heights h of the openings 9a and 10a are represented by ratios w / W and h / H with respect to the width W and depth H of the tundish molten steel surface. From the simulation results using the same calculation model, it has been found that when the inclusion outflow rate is less than 10%, the slab defect produced from the obtained slab has an acceptable reference value of 0.1% or less. Therefore, the evaluation when the inclusion outflow ratio was less than 10% was judged as “good”, and the case where the inclusion was over 10% was judged as “poor”.

As shown in FIG. 3, the portion surrounded by a dotted line is a good range where the outflow rate of inclusions is less than 10%. From this result, the condition that satisfies the following formulas (1) and (2) may be It was found that this is an appropriate condition.
1/6 ≦ w / W ≦ 2/5 (1)
− (5/4) · (w / W) + 17/24 ≦ h / H ≦ 3/4 (2)
here,
W: The width of the tundish molten steel surface,
H: Depth of tundish molten steel surface,
w: width of the openings 9a and 10a of the portion immersed in the molten steel of the weir,
h: Height of the openings 9a and 10a of the portion immersed in the molten steel of the weir

  As shown in FIG. 2, the flow pattern in the case of the suitability condition is that the molten steel flowing out from the opening 9 a at the upper part of the first weir 9 collides with the blocking part at the upper part of the second weir 10 and flows. The direction is changed, and the opening 10a of the second weir 10 is in the lower part, so that a downward flow is formed. As a result, when viewed from above, in the region surrounded by the first weir 9 and the second weir 10 It was found that vortices are often formed. Even when no vortex is formed, a short-circuit flow from the opening 9a of the first weir 9 to the opening 10a of the second weir 10 is not formed, but flows out from the opening 9a above the first weir 9. After the molten steel collides with the blocking portion at the upper part of the second weir 10, the molten steel descends along the second weir 10 to form a detour path that passes through the opening 10 a of the second weir 10. As a result, the flow path length of the molten steel can be lengthened, and the inclusions are aggregated and coalesced when colliding with the blocking portion of the second weir 10 or being caught in a vortex, and the inclusions are coarse. The progress of the conversion is also advantageous for the removal of the inclusions. Since inclusions are floating between the first and second weirs 9 and 10, the amount of inclusions contained in the molten steel flowing out from the opening 10a at the lower part of the second weir 10 is reduced. Highly clean molten steel with inclusions reduced to 10% or less of the amount introduced is injected into the mold.

  On the other hand, FIG. 4 shows a schematic diagram of the flow pattern when the width ratio w / W of the openings 9a and 10a exceeds 2/5. A short circuit flow to the opening 10a of the second weir 10 is formed, and the inclusion floating effect is reduced. FIG. 5 shows a flow pattern when the width ratio w / W is less than 1/6. In this case, a vortex is formed, but the flow velocity from the opening 9a of the first weir 9 is fast, Since the slag is caught from the molten metal surface when it collides with the blocking part at the upper part of the second weir 10, the inclusion reduction effect is offset.

  Further, even when the width ratio w / W of the openings 9a and 10a satisfies the formula (1), if the height ratio h / H exceeds 3/4, the opening of the first weir 9 is passed through the second weir. A short-circuit flow to the 10 openings is formed. Further, when the height h / H ratio is less than the value “− (5/4) · (w / W) +17/24” on the left side of the formula (2), the ratio from the opening 9a of the first weir 9 When the flow velocity is high and it collides with the interruption | blocking part of the upper part of the 2nd dam 10, slag will be caught from the hot_water | molten_metal surface. For this reason, if it is not the range which satisfy | fills Formula (2), the inclusion floating removal effect will deteriorate.

  As described above, the first weir 9 having the opening 9a in the lower portion has been described based on FIG. 2, which is an example in which the first weir 9 and the second weir 10 are arranged. Even when the position is changed and the second weir 10 is arranged on the side close to the inlet 2, the same effect can be obtained. The simulation result in this case is shown in FIG. The good range where the inclusion outflow rate is less than 10% is surrounded by a dotted line, but it is the same region as FIG. 3 showing the simulation result when the first weir 9 is close to the inlet 2. FIG. 7 shows a flow pattern often seen in a good range when the second weir 10 is arranged on the side close to the inlet 2. After the molten steel flowing out from the opening 10a of the second weir 10 collides with the blocking part at the lower part of the first weir 9 and changes direction, the result rises toward the upper part of the first weir 9, As seen from above, it has been found that vortices are often formed in the region surrounded by the second weir 10 and the first weir 9. Even when the vortex is not formed, the short circuit flow is not formed and the detour path is formed, which is the same as the case where the first weir 9 is disposed on the side close to the inlet 2. That is, even if either the first weir 9 or the second weir 10 is arranged on the side close to the inlet 2, a high inclusion floating removal effect can be obtained. However, in the simulation result, since the numerical value of the inclusion outflow rate tends to be lower when the first weir 9 having the opening 9a on the upper side is arranged closer to the inlet 2, the first weir 9 is It is preferable to arrange it on the side close to the inlet 2.

  As described above, in the region between adjacent weirs, it is possible to form a vortex with a slight rotation or a detour path to increase the flow path length and to promote the aggregation and coalescence of inclusions. Therefore, (a) two or more weirs are provided, and (b) in at least two adjacent weirs 9, 10, the positions of the openings 9a, 10a in the height direction are the first weir 9 In the upper portion, in the second dam 10 in the lower portion, and (c) the widthwise positions of the openings 9a, 10a in the dam are on the one side of the side wall of the tundish 3 in the first dam 9. The second weir 10 is located on the opposite side of the first weir 9, and (d) the widths w and heights h of the openings 9a, 10a of the respective weirs 9, 10 are expressed by the equations (1) and (2). It is necessary to satisfy.

  Furthermore, it is more effective to provide three or more weirs. In this case, the odd-numbered weir blocks the molten steel flow in the lower part of the molten steel bath in the height direction, blocks the molten steel flow on one side of the side wall in the upper part, and on the other side of the upper part. The even-numbered weir blocks the molten steel flow in the upper part in the height direction of the molten steel bath, and the lower part has an opening provided above the odd-numbered weir. The flow of the molten steel on the same side is cut off and an opening is formed on the other side of the lower part, and the width and height of the opening of each portion immersed in the molten steel are w, h and (1) and (2) are satisfied. That is, it is close to the form in which the first weir 9 and the second weir 10 are alternately arranged in the case of the two weirs described above. However, as long as the width and height of the opening satisfy the expressions (1) and (2), it is not necessary that the odd-numbered or even-numbered parts have the same shape. FIG. 8 shows an example of the flow pattern when three dams are arranged. The shapes of the openings 9a and 11a of the first dam 9 and the third dam 11 satisfy the expressions (1) and (2). As long as it is different. This makes it possible to form a vortex or detour path in the area between adjacent weirs to increase the length of the flow path, and to promote the aggregation and coalescence of inclusions. can get.

  In FIG. 8, the first weir 9 with the opening 9 a located above is arranged on the side close to the injection port 2. The process up to the opening 10a of the second weir 10 is the same as the case where the two weirs described above are provided. The molten steel flow coming out of the opening 10a at the lower part of the second weir 10 collides with the blocking part at the lower part of the opposite third weir 11 to change the flow direction. As a result, the tundish 3 is viewed from above. In some cases, a vortex or a detour path is formed in the region between the second weir 10 and the third weir 11. The vortex formed in the region between the first weir 9 and the second weir 10 described above is opposite in rotation direction, and rises toward the opening 11a in the upper part of the third weir 11 while rotating strongly for one rotation. Current. Therefore, in addition to the increase in the flow path length when the number of the weirs is two and the promotion of the aggregation and coalescence of inclusions, the effect of promoting the floating removal of inclusions by this upward flow is also added. The inclusion floating removal effect can be further enhanced compared to the case of two weirs, and the inclusion outflow ratio has been reduced to 3% or less in the simulation results.

  The example of the embodiment of the present invention has been described above, but the present invention is not limited to this example and can take various forms. In the illustrated embodiment, an example has been described in which two or three weirs are provided between the tundish injection portion and the outlet. However, the number of weirs is arbitrary as long as it is two or more. In addition, in at least two adjacent weirs among the plurality of weirs, an opening is formed in one weir that allows molten steel to flow only in the direction of the side wall on the upper side of the molten steel bath. It is only necessary to form the opening only in the side wall direction opposite to the opening in the lower part of the molten steel bath in the height direction. In FIG. 2 and the like, the widths and heights of the openings 9a, 10b, and 11a have been described using the common symbols w and h. However, the widths and heights of the openings formed in the weirs. h may be the same as each other, and the width w and height h of the opening may be different for each weir. In any case, it is sufficient that the width w and height h of the opening formed in at least two adjacent weirs satisfy the conditions of the expressions (1) and (2) described above.

Hereinafter, the present invention will be described in detail with reference to examples.
Low carbon steel 300t was melted in a converter and aluminum deoxidized, and then cast by a 2-strand vertical bending type continuous casting machine through a boat-type tundish with a molten steel capacity of 60t, and the width was 1800mm x thickness 280mm. A slab was produced. In order to investigate the amount of alumina inclusions in the slab, samples from the slab surface layer to the 80 mm depth from the slab surface were collected from the 1/4 and 1/2 width parts in the slab width direction. The number of alumina inclusions having a size exceeding 53 μm was measured.

  In the tundish, 0 to a maximum of 3 weirs were provided. The shape of the weir is an inverted trapezoid according to the tundish cross section, the height of the portion immersed in the molten steel during casting is 1400 mm, the width of the molten steel bath surface is 1400 mm, the width of the bottom is 800 mm, and the thickness is 100 mm.

  Table 1 shows the tundish specifications and the inclusion investigation results as examples of the present invention (examples of the present invention) and comparative examples. The first and third weirs have an opening at the top and the second weir at the bottom. In addition, the side on which the first weir was placed on the inlet side or the outlet side was described. For example, if the position of the first weir is described as the inlet side, it indicates that the first weir, the second weir, and the third weir are arranged in order from the inlet side.

  When the number of alumina inclusions measured in the slime survey is 50 pieces / kg or less, it is known that the product defect is the target value of 0.1% or less. Was determined to be x, and the case of exceeding 50 / kg was determined to be x.

  No. in Table 1 1-No. 5 is a comparative example. No. 1 is No. when there is no weir. 2 is the width ratio w / W = 1 of the openings of all the weirs, that is, in the case of a conventional three-stage weir in which two lower weirs and one upper weir are arranged over the entire width, No. 3 was a case where there was one weir, and the result was a bad result of more than 100 inclusions / kg. Evaluation is x.

  No. 4 and no. 5 is a case where two weirs do not satisfy the present invention range of the height ratio h / H of the opening. No. In No. 4, slag entrainment on the hot water surface occurred. In No. 5, the short circuit flow was formed, so the effect of inclusion floating was low, and the number of inclusions exceeded 50 / kg. Evaluation is x.

  No. 6 to 17 are examples of the present invention. No. Nos. 6-10 have two weirs and the first and second weirs have the same opening shape. 11-14 is a case where the number of the weirs is two and the shape of the opening part of a 1st weir and a 2nd weir differs. And no. 6-No. No. 13 is the first weir on the inlet side, Reference numeral 14 denotes a case where the first weir is arranged on the outlet side. In both cases, the width / width ratio w / W and the height ratio h / H of the opening satisfy the scope of the present invention. In all cases, the floating removal effect of inclusions was good, and the number of inclusions was 50 / kg or less. Evaluation is (circle).

  No. 15-No. Reference numeral 17 denotes an example of the present invention when there are three weirs. No. No. 15 is the case where all the three weir openings have the same shape. 16 and No. Reference numeral 17 denotes a case where the shape of the opening of each weir is different. And no. 15 and No. No. 16 has the first weir on the inlet side, 17 arrange | positions the 1st weir on the outflow port side. In both cases, the number of inclusions was 20 / kg or less, which was even better than when there were two weirs. Evaluation is (circle).

  As described above, in the tundish according to the present invention, by defining the position and shape of the opening of the weir, a vortex or a detour path is formed without forming a short-circuit flow in the region between the weirs. As a result, the flow path length of the molten steel is lengthened, and further, aggregation and coalescence of inclusions are promoted, so that the inclusion floating removal effect is high. As a result, molten steel with high cleanliness is injected into the mold, and the alumina inclusions remaining in the cast slab can be greatly reduced, reducing the surface flaws of the product and cracking during processing. , Industrial effect is great.

  The tundish of the present invention can be used for continuous casting of steel.

It is explanatory drawing of a prior art. It is the figure which looked at the figure which looked at the tundish for continuous casting concerning the embodiment of the present invention which arranged two sheets of weirs on the injection port side, and the side from the top, and shows a flow pattern typically Show. It is a simulation result of embodiment of this invention which arranged the 1st weir on the injection port side with two dams, the inclusion outflow rate and the width ratio of the opening; w / W and height ratio; h / H It is a graph which shows the relationship. It is a schematic diagram of a flow pattern in case two weirs and the width ratio of an opening part; w / W exceeds 2/5. It is a schematic diagram of the flow pattern in the case where there are two weirs and the width ratio of the opening; w / W is less than 1/6. It is a simulation result of embodiment of this invention which arranged the 2nd weir on the injection port side with two weirs, the inclusion outflow ratio and the opening width ratio; w / W and height ratio; h / H It is a graph which shows the relationship. It is the figure which looked at the figure which looked at the tundish for continuous casting concerning the embodiment of the present invention which arranged two dams on the injection port side from the top, and from the side, and shows a flow pattern typically. Show. It is the figure which looked at the figure which looked at the continuous casting tundish concerning the embodiment of the present invention which arranged three dams and the 1st weir on the injection mouth side from the top, and the flow pattern. Show.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ladle 2 Inlet 3 Tundish 4 Outlet 5 Immersion nozzle 6 Lower weir 7 Upper weir 8 Lower weir 9 First weir 10 Second weir 11 Third weir 9a, 10a, 11a Opening W: Tundish Width of molten steel surface,
H: Depth of tundish molten steel surface,
w: width of the opening of the portion of the weir immersed in the molten steel,
h: Height of the opening of the portion of the weir immersed in the molten steel

Claims (2)

A tundish in continuous casting of steel, having a plurality of weirs between an injection part from a ladle and an outlet to a mold, and one of at least two adjacent weirs among the plurality of weirs Has an opening through which molten steel can circulate only in the side wall direction on the upper side of the molten steel bath, and the other weir is opened only in the side wall direction opposite to the opening in the lower portion of the molten steel bath height direction. The tundish for continuous casting, wherein each weir satisfies the following formulas (1) and (2).
1/6 ≦ w / W ≦ 2/5 (1)
− (5/4) · (w / W) + 17/24 ≦ h / H ≦ 3/4 (2)
here,
W: The width of the tundish molten steel surface,
H: Depth of tundish molten steel surface,
w: width of the opening of the portion of the weir immersed in the molten steel,
h: Height of the opening of the portion of the weir immersed in the molten steel   A weir having an opening through which molten steel can flow only in the direction of the side wall on the upper side of the molten steel bath is arranged on the side close to the molten steel injection portion from the ladle, and the lower portion of the molten steel bath in the height direction of the molten steel bath 2. The tundish for continuous casting according to claim 1, wherein a weir having an opening only in a side wall direction opposite to the opening is arranged on the side close to the outflow port.

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