Tundish and continuous casting method using the same

Abstract

Provided is a technique capable of efficiently and inexpensively promoting the floatation of inclusions. This tundish comprises a weir having a wall that surrounds a molten steel pouring section, in which a molten steel pouring flow from a ladle collides with the tundish bottom, on four sides and extends upward from the tundish bottom between the molten steel pouring section and a molten steel outlet from the tundish to a mold, and eaves protruding horizontally toward the molten steel pouring section at the upper end of the wall. The weir has one or more cutouts continuous from the wall across the eaves, has a first porous portion in a refractory bottom surrounded by the wall, and has a first gas introduction pipe connected to the first porous portion within the weir and the refractory including the first porous portion. The tundish also optionally comprises a bottom refractory having a second porous portion and a second gas introduction pipe connected to the second porous portion in the tundish bottom between the weir and the molten steel outlet.

Description

Technical Field

[0001] The present invention relates to a tundish for relaying molten metal injected from a ladle in a continuous casting process and supplying it to a mold, and a tundish capable of efficiently removing non-metallic inclusions in the molten metal injected into the tundish and a continuous casting method using the same.

Background Art

[0002] For the production of high-quality steel materials, further improvement in the high cleanliness technology of molten steel is required. Non-metallic inclusions such as Al2O3 in molten steel, which are deoxidation products, cause defects after rolling, so it is necessary to separate and remove them as much as possible in the casting stage. The tundish serves to float and separate non-metallic inclusions that flow out together with the molten steel when pouring the molten steel from the ladle. The higher the floating separation ratio, the more possible it is to produce high cleanliness molten steel.

[0003] So far, methods for efficiently floating and separating inclusions in a tundish have been proposed. For example, in Patent Document 1, a porous weir extending from the bottom to a position higher than the molten metal surface is provided in the tundish, which divides the tundish into a molten metal receiving area from the ladle and a quasi-stationary area of steel having an outlet to the mold, and the injection nozzle from the ladle is immersed in the molten metal in the receiving area to supply the molten metal. A technique for promoting the floating effect of inclusions is disclosed by a method for producing clean steel.

[0004] In Patent Document 2, there is disclosed a tundish divided into a molten metal receiving side and an outlet side by a porous weir having holes in contact with the bottom wall, and a lower weir with an open upper part is installed on the outlet side of the porous weir, and a technique for optimizing the shape of the tundish, the position of the weir, the hole shape, and the hole position is disclosed.

[0005] Further, in Patent Document 3, a weir having a flow hole is installed in the middle of the injection position from the ladle and the outlet to the mold, and a predetermined amount of inert gas is blown from the bottom of the tundish on the outlet side of the weir to enhance the inclusion floating effect. A technique is disclosed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the above prior art has the following problems. In the technology described in Patent Document 1, a hole for removing residual steel is provided on the bottom side of the tundish of the perforated weir. After inclusions in the molten steel pass near the bottom of the perforated weir, there is a concern that the inclusions may flow out due to a short-circuit flow that flows from near the bottom of the tundish toward the molten steel outlet.

[0008] In the technology of Patent Document 2, inclusions in the molten steel are promoted to float by the lower weir after passing through the perforated weir, but the floating effect is insufficient. In addition, since molten steel remains on the steel receiving side from the lower weir after the casting is completed, there is a concern that the cost of residual steel will increase.

[0009] In the technology of Patent Document 3, although the inclusion floating effect is enhanced by blowing an inert gas, the effect on inclusion floating is small unless the gas volume ratio in the molten steel is increased. On the contrary, if the gas flow rate is increased, the molten steel surface will fluctuate when the bubbles burst, and there is a concern about molten steel contamination due to entrainment of tundish slag present on the molten steel surface.

[0010] The present invention has been made in view of the above circumstances, and an object thereof is to provide a tundish capable of efficiently and inexpensively promoting the floating of inclusions contained in molten steel injected from a ladle into the tundish, and to propose a continuous casting method using the tundish.

Means for Solving the Problems

[0011] The tundish according to the present invention for solving the above problems has a wall portion that surrounds the molten steel injection portion from four directions and extends upward from the bottom of the tundish between the molten steel injection portion where the molten steel injection flow from the ladle collides with the bottom of the tundish and the molten steel outlet from the tundish to the mold, and a shielding portion that protrudes horizontally toward the molten steel injection portion side at the upper end of the wall portion. The weir has at least one continuous notch extending from the wall portion to the shielding portion, has a first porous portion at the refractory bottom surrounded by the wall portion, and has a first gas introduction pipe connected to the first porous portion in the weir and in the refractory including the first porous portion. Further, optionally, a bottom refractory having a second porous portion and a second gas introduction pipe connected to the second porous portion is provided at the bottom of the tundish between the weir and the molten steel outlet.

[0012] In addition, the tundish according to the present invention further preferably has a third gas introduction pipe connected to or extending from the first gas introduction pipe or the second gas introduction pipe inside, and includes a precast refractory constructed on the wall portion of the tundish.

[0013] Also, the continuous casting method according to the present invention uses the above tundish, adjusts the inert gas flow rate R1 per unit area at the bottom of the weir to be in the range of 0.02 to 1.0 NL / (s·m 2 ), and while blowing an inert gas into the molten steel from the first porous portion through the first gas introduction pipe, casts from the tundish into the mold to continuously cast a steel slab.

[0014] In addition, the continuous casting method according to the present invention further preferably adjusts the inert gas flow rate R2 per unit area to be in the range of 0.1 to 10 NL / (s·m 2 ), and blows an inert gas into the molten steel from the second porous portion through the second gas introduction pipe.

Advantages of the Invention

[0015] According to the tundish of the present invention, it is possible to suppress the short-circuit flow of the molten steel injected from the ladle flowing at the bottom of the tundish, change the flow upward, and promote the floating separation of inclusions. In addition, the floating separation of inclusions can be promoted by the upward movement of bubbles from the porous part of the weir.

[0016] In addition, the tundish according to the present invention is preferably such that bubbles blown into the molten steel from the porous part provided in the bottom refractory between the weir and the molten steel outlet can further promote the floating separation of inclusions before flowing out into the mold.

[0017] Furthermore, the tundish according to the present invention further has a gas introduction pipe inside and is provided with precast refractories constructed on the wall of the tundish, so that a device for blowing gas from the porous parts installed at the bottom of the weir and the bottom of the tundish can be easily constructed. Therefore, it is possible to suppress the operation hindrance due to construction defects, there is no risk of steel leakage, etc., and it is safe and preferable.

[0018] According to the continuous casting method of the present invention, since the above tundish is used and the amount of inert gas blown from the bottom of the weir and the bottom of the tundish is set within an appropriate range, it is sufficient for the floating separation of inclusions and can suppress the entrainment of tundish slag from the molten steel surface. Therefore, high-purity steel can be easily produced.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be specifically described. Note that each drawing is schematic and may be different from the actual one. Further, the following embodiments illustrate devices and methods for embodying the technical idea of the present invention, and do not specify the configuration to be the following. That is, the technical idea of the present invention can be variously modified within the technical scope described in the claims.

[0021] A tundish according to an embodiment of the present invention is schematically shown in FIG. 1. FIG. 1(a) is a sectional view taken along line A-A, FIG. 1(b) is a sectional view taken along line B-B, and FIG. 1(c) is a sectional view taken along line C-C. The tundish 1 of the present embodiment is an intermediate container used for continuous casting of steel, for injecting molten steel in a ladle into a mold for continuous casting. The tundish 1 is, for example, a substantially rectangular parallelepiped container with an open upper surface. Molten steel is supplied from a ladle (not shown) through an injection nozzle 2. In the example of FIG. 1, the stored molten steel is supplied to a mold (not shown) from two molten steel outlets 3 provided at the bottom, respectively.

[0022] In this embodiment, a weir 4 is provided between a molten steel injection part 2a where a molten steel injection flow from a ladle collides with the bottom 1a of a tundish and a molten steel outlet 3 from the tundish 1 to a mold. The weir 4 has a wall part 4a that surrounds the molten steel injection part 2a from four directions and extends upward from the bottom 1a of the tundish, and a eaves-like part 4b that protrudes horizontally toward the molten steel injection part 2a side at the upper end of the wall part 4a. The weir 4 has one or more continuous notches extending from the wall part 4a to the eaves-like part 4b. The weir 4 has a first porous part 4d in a refractory bottom 4c surrounded by the wall part 4a, and has a first gas introduction pipe 5a connected to the first porous part 4d in the weir and in the refractory including the first porous part 4d. The first porous part 4d preferably occupies 15% or more of the total area of the refractory bottom 4c surrounded by the wall part 4a of the weir 4. Although there is no upper limit defined, it is preferably not installed near the collision point of the molten steel injection flow from the ladle.

[0023] With such a configuration, the short-circuit flow of the molten steel injected from the ladle flowing through the bottom 1a of the tundish can be suppressed, and by changing the flow upward, the floating separation of inclusions can be promoted. Furthermore, by blowing an inert gas from the first porous part 4d, non-metallic inclusions can be captured by the inert gas bubbles, and the floating separation can be further promoted. By installing the first porous part 4d in the refractory bottom 4c of the molten steel injection part 2a, the gas bubbles that detach from the first porous part 4d are refined by the shearing force of the high-speed injection flow that collides with the refractory bottom 4c and travels horizontally, and the effect of increasing the capture probability of inclusions can be obtained.

[0024] Fig. 2 shows a graph of the influence of the inert gas flow rate R1 [NL / (s·m 2 )] per unit area at the bottom of the weir 4 on the number of inclusions flowing out to the steel slab. The number of inclusions flowing out to the steel slab was evaluated by the number of inclusions per unit area through microscopic observation after taking 5 samples from the two widest surfaces of the rectangular parallelepiped slab for the inclusions of 10 μm or more in the slab and polishing the observation surface. As is clear from Fig. 2, when R1 is less than 0.02 NL / (s·m 2 )), the inclusion floating effect in the tundish is small, which is not preferable. On the other hand, when R1 is 1.0 NL / (s·m2 ) exceeds, the amount of gas blown in is too large, and the entrainment of tundish slag increases, which is not preferable. Therefore, the flow rate R1 of the inert gas per unit area at the bottom of the weir 4 is 0.02 to 1.0 NL / (s·m 2 ) needs to be adjusted to be in the range. Preferably, the inert gas flow rate R1 is 0.02 to 0.2 NL / (s·m 2 ) is in the range.

[0025] The supply of the inert gas to the first porous part 4d is preferably provided with a precast refractory 6 that is connected to the first gas introduction pipe 5a provided in the weir 4 or has a third gas introduction pipe 5c extending from the first gas introduction pipe 5a inside and is constructed on the wall part of the tundish 1. By doing so, the construction of the refractory in the tundish becomes simple, and it is possible to suppress the operation hindrance due to construction defects.

[0026] In the present embodiment, further, optionally, a refractory 7 having a second porous part 7a at the bottom 1a of the tundish between the weir 4 and the molten steel outlet 3 and a second gas introduction pipe 5b connected to the second porous part 7a is preferably provided. It is preferable to have a third gas introduction pipe 5c connected to the second gas introduction pipe 5b or extending the second gas introduction pipe 5b inside and be provided with a precast refractory 6 constructed on the wall part of the tundish 1. The refractory 7 and the precast refractory 6 may be integrally formed. The refractory 7 is preferably installed across the entire bottom 1a of the tundish in a direction perpendicular to the flow of the molten steel in which the molten steel injection flow from the ladle heads toward the molten steel outlet 3 to the mold. Similar to the lower weir, a flow toward the tundish surface can be generated to promote the floating separation of inclusions. In addition, as shown in FIG. 1, an upper weir 8 may be installed upstream of the installation position of the refractory 7 having the second porous part 7a, that is, on the molten steel receiving side from the ladle. It is possible to prevent the inclusions floating on the molten steel receiving side from flowing out to the injection side to the mold.

[0027] In FIG. 3, the inert gas flow rate R2 [NL / (s·m per unit area of the second porous part 7a of the refractory 7 2)] is shown in the relationship graph between the number density of inclusions in the slab. In Fig. 3, the test was conducted without blowing inert gas from the first porous part 4d installed in the dam 4. The evaluation of inclusions was carried out in the same manner as described above. As is clear from Fig. 3, when R2 is less than 0.1 NL / (s·m 2 ), the inclusion floating effect in the tundish is small, which is not preferable. On the other hand, when R2 exceeds 10 NL / (s·m 2 ), the amount of gas blown is too large, and the entrainment of tundish slag increases, which is not preferable. Therefore, it is preferable to adjust the flow rate R2 of the inert gas per unit area of the second porous part 7a to be in the range of 0.1 to 10 NL / (s·m 2 ).

[0028] It is preferable to integrally mold the refractory 7 including the second porous part 7a with the precast refractory 6 constructed on the wall part of the tundish 1, which can shorten the maintenance time of the tundish.

[0029] The first porous part 4d and the second porous part 7a can be produced by using spherical particles mainly composed of alumina as aggregates and firing at 1600 °C or higher. The average pore diameter of the first porous part 4d and the second porous part 7a is preferably 20 to 120 μm. The average pore diameter can be defined, for example, by the mercury intrusion method in accordance with JIS R 1655:2003. By setting the average pore diameter within this range, the bubble diameter blown into the molten steel can be controlled within a predetermined range, which is effective for suppressing slag entrainment.

Example

[0030] 300 t of molten steel, which was oxygen blown in a converter and vacuum degassed in an RH type vacuum degassing device, was accommodated in a ladle. It was continuously cast by injecting it from the ladle through the tundish 1 shown in Fig. 1 into a mold. In the tundish 1, under the conditions shown in Table 1, the flow rate R1 [NL / (s·m 2 )] of the inert gas per unit area at the bottom of the dam 4 and the flow rate R2 [NL / (s·m 2)] was adjusted. The inclusion number density in the slab after each treatment was investigated in the same manner as above. The results are shown graphically in Fig. 4.

[0031]

Table 1

[0032] Treatment No. 1 is a conventional example where R1 and R2 are 0. Treatment Nos. 2 to 4 are inventive examples where an appropriate amount of inert gas is blown only from the first porous part 4d at the bottom of the weir 4. Treatment Nos. 5 to 8 are reference examples where an appropriate amount of inert gas is blown only from the second porous part 7a installed at the tundish bottom between the weir 4 and the molten steel outlet 3. Treatment Nos. 9 to 12 are inventive examples where an appropriate amount of inert gas is blown by combining both. Treatment Nos. 13 to 16 are comparative examples where the blowing amount of the inert gas is outside the appropriate range. From the results in Fig. 4, it can be seen that in the inventive examples where the inert gas is blown within an appropriate range, the cleanliness of the slab is significantly improved compared to the conventional examples and the comparative examples.

[0033] In this specification, the unit of volume "L" means 10 -3 m 3 and the symbol "N" indicating the volume of gas represents the volume at a temperature of 0°C and a pressure of 101325 Pa under standard conditions.

Explanation of Symbols

[0034] 1 Tundish 1a Tundish bottom 2 Injection nozzle 2a Molten steel injection part 3 Molten steel outlet 4 Weir 4a Wall part 4b Canopy part 4c Refractory bottom 4d (First) porous part 5a (First) gas introduction pipe 5b (Second) gas introduction pipe 5c (Third) gas introduction pipe 6 Precast refractory 7 Refractory (including the porous part) 7a (Second) porous part 8 Upper weir

Claims

Claim 1 Between a molten steel injection part where a molten steel injection flow from a ladle collides with the bottom of a tundish and a molten steel outlet from the tundish to a mold, there is a weir having a wall part that surrounds the molten steel injection part from four directions and extends upward from the bottom of the tundish, and a eaves-like part that protrudes horizontally facing the molten steel injection part side at the upper end of the wall part. The weir has one or more notches continuous from the wall part to the eaves-like part, has a first porous part at the refractory bottom surrounded by the wall part, and has a first gas introduction pipe connected to the first porous part in the weir and in the refractory including the first porous part. The tundish. Claim 2 Furthermore, it includes a precast refractory constructed on the wall of the tundish, which has a third gas introduction pipe connected to or extended from the first gas introduction pipe inside. The tundish according to claim 1. Claim 3 The tundish according to claim 1, comprising a bottom refractory having a second porous part and a second gas introduction pipe connected to the second porous part at the bottom of the tundish between the weir and the molten steel outlet. Claim 4 Furthermore, it includes a precast refractory constructed on the wall of the tundish, which has a third gas introduction pipe connected to or extended from the first gas introduction pipe or the second gas introduction pipe inside. The tundish according to claim 3. Claim 5 Using the tundish according to any one of claims 1 to 4, adjusting the flow rate R1 of the inert gas per unit area at the bottom of the weir to be in the range of 0.02 to 1.0 NL / (s·m ２ ), while blowing an inert gas from the first porous part into the molten steel through the first gas introduction pipe, continuously casting a steel slab by casting from the tundish into a mold. **Claim 6**: Using the tundish according to claim 3 or 4, adjusting so that the flow rate R1 of the inert gas per unit area at the bottom of the weir is in the range of 0.02 to 1.0 NL / (s·m2), blowing an inert gas from the first porous part into the molten steel through the first gas introduction pipe, and further adjusting so that the flow rate R2 of the inert gas per unit area is in the range of 0.1 to 10 NL / (s·m ２ ), while blowing an inert gas from the second porous part into the molten steel through the second gas introduction pipe, casting from the tundish into a mold to continuously cast a steel slab, a continuous casting method.

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