JP4220848B2 - Tundish for continuous casting of steel with heating function - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a tundish for continuous casting of steel.
[0002]
[Prior art]
In the continuous casting of steel, the molten steel whose components and temperature are adjusted in the refining process is transported to the continuous casting process using a ladle. The transported molten steel is injected into the mold of a continuous casting machine using a refractory nozzle, but it is difficult to control the flow rate of the molten steel directly from the ladle. The molten steel is temporarily stored in an intermediate vessel called a tundish through a nozzle or a shroud sealed with an inert gas that suppresses oxidation, and the molten steel is placed in the mold through the nozzle while controlling the flow rate. It is common to supply.
[0003]
There are various types of tundish, but most commonly, molten steel is supplied from a ladle through a nozzle to the center of the boat-shaped tundish, and the tundish is placed at a location corresponding to the two tips of the boat. The flow rate is controlled by controlling the opening area of the hole with a rod-like stopper that controls the flow rate by controlling the cross-sectional area of the outlet by moving up and down, or two to three plates with round holes. The sliding nozzle supplies molten steel from both ends to two continuous casters (called strands).
[0004]
In addition to having the function of pouring molten steel from one ladle into two molds as described above, tundish also has a function of dropping slag, which is an oxide that is inevitably mixed when refining steel. So-called non-metallic inclusions such as alumina that are usually produced to perform acid are floated and separated using the fact that their specific gravity is less than half of the specific gravity of steel, and penetrates into the mold and is captured by the steel during rolling. It also has a function to prevent generation of soot.
[0005]
As described above, inclusions are levitated by utilizing the difference in specific gravity, but there are steels with diameters from several μm to millimeters in the steel. There is a growing demand for improvement in flying characteristics. Therefore, as an attempt to control the flow by providing a weir in the same tundish to increase the floating separation rate, for example, (Patent Document 1) or (Patent Document 2) increases the floating property of nonmetallic inclusions. Therefore, a technique for arranging a plurality of flat plate-like weirs is disclosed.
[0006]
As mentioned above, the slag reservoir formed by the large inclusions in the molten steel injected from the ladle is collected around the nozzle that supplies the molten steel from the ladle to the tundish, so that it does not move toward the mold. Many of the upper weirs and the lower weirs provided so that the molten steel poured from the ladle into the tundish does not directly flow toward the mold are used as basic components.
[0007]
On the other hand, the tundish has the above functions, but the temperature of the molten steel gives an additional temperature called superheat to the so-called liquidus temperature where solidification starts in the mold, and in the ladle, tundish, and nozzle. It is customary to prevent the molten steel from solidifying.
[0008]
However, since the temperature of the molten steel poured into the tundish from the ladle gradually decreases during the pouring, a technique has been invented in which heat is applied to the molten steel in the tundish to suppress the temperature drop. For example, as described in (Non-Patent Document 1), there is a method of heating molten steel using thermal plasma.
[0009]
In this technique, a potential difference is applied between the electrode and the molten steel, plasma is generated, and the molten steel is heated by the Joule heat and radiant heat. In this heating method, since a part of the molten steel is heated, it is also described that gas agitation is used for equalization.
[0010]
[Patent Document 1]
JP-A-1-224152 [Patent Document 2]
JP-A-7-132353 [Non-patent Document 1]
129, 130th Nishiyama Memorial Technology Course Material Processing Using Electromagnetic Force Japan Iron and Steel Institute April 28, 1989 Page 247
[0011]
[Problems to be solved by the invention]
However, in (Non-patent Document 1), since gas agitation is required, it is necessary to provide a gas blowing port for porous bricks in the tundish. Due to the turbulence of the flow, there has been a problem that inclusions floating in the vicinity of the molten steel surface in the tundish are brought into the inside again, or oxide floating in the vicinity of the molten steel is entrained to contaminate the molten steel.
An object of this invention is to provide the tundish which improved the temperature control and the cleaning function of molten steel.
[0012]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) In the tundish arranged between the ladle for transporting molten steel from the refining process and the continuous casting mold, the nozzle position for injecting molten steel from the ladle to the tundish, and the molten steel from the tundish to the mold and Kamizeki pair between nozzle position for injecting, the lower weir is disposed at a position sandwiched Kamizeki of the pair, and plasma heating electrode is disposed between Kamizeki of the pair, the downstream side The distance D (m) between the lower end of the upper weir and the bottom of the tundish, the molten steel depth H (m), the molten steel flow channel average width W (m), the molten steel throughput Q (t / min), A tundish for continuous casting of steel having a heating function, wherein the distance L (m) from the upper weir on the flow side to the molten steel outlet satisfies the following formula (1) .
L> α × { Q × (H + D / 2) ^1/2 } / (D × W) (1)
Where α: γ / { ρ × (β × g × ΔT) ^1/2 }
ρ: Molten steel density at the liquidus temperature of molten steel (kg / m ³ )
β: Body expansion coefficient of molten steel (1 / K)
g: Gravity acceleration
ΔT: Average molten steel temperature rise due to plasma heating (K)
γ: a proportionality constant determined by operating conditions .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The apparatus and operation of the present invention will be described below with reference to the drawings.
FIG. 1 shows an example of a tundish according to the present invention. The molten steel transported by the ladle 1 through the refining process is injected into the tundish 3 through the injection nozzle 2 from the ladle. The injected molten steel is transported in the tundish to a molten steel outlet 4 connected to an injection nozzle for a mold. That is, the molten steel poured into the tundish 3 from the ladle moves through the upper weir 5, the lower weir 6, and the upper weir 7 to the molten steel outlet 4 leading to the casting nozzle into the mold.
[0014]
Here, the upper weir 5 acts to suspend large inclusions and slag droplets flowing from the ladle 1, holding them on the molten steel surface as they are, and preventing them from flowing out to the mold side.
Next, the molten steel passing through the upper weir 5 collides with the lower weir 6 to form an upward flow. Further, a downward flow is created in front of the upper weir 7 and then flows out to the mold side.
[0015]
Generally, when there is no heating, the molten steel receives heat from the surface of the molten steel in the process of passing through the tundish. Accordingly, it is known that the molten steel shrinks and becomes heavy on the upper molten metal surface and the side of the wall and sinks downward, creating so-called thermal convection.
On the other hand, when heating is performed with a normal tundish in which the weir of the present invention is not installed, only the molten steel on the molten metal surface side being heated is heated, and only the molten steel in this portion expands. Since it becomes lighter, heat convection hardly occurs.
[0016]
Therefore, when the apparatus of the present invention is used, the molten steel heated between the upper weirs is forced to move to the tundish bottom side by the upper weirs by heating with a plasma 8 between the pair of upper weirs 5 and 7. Since extrusion and heat supply from the lower part, it is possible to suppress the temperature drop of the molten steel in the tundish and to keep the molten steel temperature substantially uniform.
[0017]
Next, when such an upper weir 7 is installed, inclusions that could not float on the upstream side (the injection nozzle side from the ladle) may be transported to the tundish bottom. That is, when it cannot float on the downstream side from the upper weir 7 (on the side of the molten steel outlet to the mold), it may be mixed in the mold and trapped in the slab to generate a defect. Therefore, it is preferable that the distance from the upper weir 7 to the molten steel outlet 4 to the mold has a sufficient distance for floating the inclusions. This distance is determined by the thermal convection of the inclusion and the rising speed of the inclusion.
[0018]
In a normal size tundish, the size of inclusions that can float and contaminate molten steel is about 50 to 100 μm in diameter, and the flying speed is the Allen equation (for example, “mechanical engineering”). It is generally known that it is expressed in “Handbook P8-20 Revised 6th edition published in 1979”).
The levitation speed is about 0.1 to 0.02 (m / s) and the speed of thermal convection is about 0.05 to 0.1 (m / s), so the levitation is greatly influenced by thermal convection. I understand that
Here, buoyancy due to temperature change = ρ · β · g · ΔT.
Where ρ is the molten steel density (kg / m ³ ) at the liquidus temperature of the molten steel.
β is the expansion coefficient of molten steel (1 / K)
g is the acceleration of gravity ΔT is the average molten steel temperature rise (K) due to plasma heating.
[0019]
Further, the inertia on the molten steel side can be expressed by kinetic energy = 1/2 · ρ · w ² .
Therefore, if the kinetic energy is equal to the product of force and travel distance,
1/2 · ρ · w ² = (ρ · β · g · ΔT) · H ′
The velocity w of thermal convection can be simply expressed as being proportional to (β · g · ΔT · H ′) ^1/2 .
Here, H ′ = H + D / 2
w is a molten steel flow velocity (m / s).
[0020]
From the above, the time obtained by dividing the distance H ′ from the position at a height of D / 2 from the bottom of the tundish to the molten metal surface by the speed of the thermal convection, that is, the time until the thermal convection reaches the molten metal surface. If the average flow velocity Q / (60 · ρ · D · W) of the molten steel exiting the upper weir 7 is shorter than the time obtained from the value obtained by dividing the distance L from the upper weir 7 to the mold position, The inclusions can be considered to rise, and the following formula is obtained from the relationship.
L> α {Q · (H + D / 2) ^1/2 } / (D × W) (1)
Where α is γ / {ρ (β · g · ΔT) ^1/2 }
Where D is the distance (m) between the lower end of the upstream weir and the bottom of the tundish
H is the depth of the molten steel at the part (m)
W is the average width of the molten steel channel (m)
Q is the throughput of molten steel (ton / min)
L is the distance between the upper weir on the wake side and the molten steel outlet (m)
ρ is the molten steel density at the liquidus temperature of the molten steel (kg / m ³ )
β is the expansion coefficient of molten steel (1 / K)
g is the gravitational acceleration ΔT is the average temperature rise of molten steel due to plasma heating (K)
γ is a proportionality constant determined by operating conditions.
However, the above W (molten steel channel average width) is
(Vertical cross-sectional area of the tundish channel through which molten steel passes) / (Maximum depth of the channel)
Defined by
[0021]
【Example】
In the structure shown in FIG. 2, in the tundish for two strands having a length of 7 (m), an average width of 1 (m), and a depth of 1 (m), the distance D between the upstream dam 7 and the bottom of the tundish is The horizontal distance L between the wake upstream dam 7 and the molten steel outlet 4 to the mold is set to 0.3 (m), the average width W 0.7 (m), and the position of the wake upstream dam 7 is set to 1.5. The test was carried out with three levels (m), 2.0 (m) and 2.5 (m). The plasma heating conditions were set to 500 (kW) for input.
[0022]
The inclusions in the molten steel were evaluated by extracting only the non-metallic inclusions by electrolytic extraction from a molten steel sample 100 (g) collected at a position of 1/4 width, center in the thickness direction, and depth of 10 cm. The number of inclusions having a diameter between 75 μm and 125 μm was investigated as representing the inclusions having a diameter of 100 μm.
Here, some of the inclusions include powder that is a lubricant in the mold, but the inclusions that do not contain sodium based on whether or not sodium is present are tundished. Only the number of the inclusions brought in was counted for comparison.
[0023]
In each case, the mold was made the same conditions with a width of 1500 (mm), a thickness of 250 (mm), and a casting speed of 1.5 (m / min). In addition, the sampling time is unified by 10 minutes from the start of casting, and it is confirmed by electrolytic extraction of inclusions from the sample taken at the entrance of the tundish that the cleanliness that is the base of the molten steel is almost the same. did.
In addition, as a comparative example, a normal tundish in which no weir nor plasma heating electrode is installed is used, and the position of the molten steel outlet 4 is the same as the case where the position of the upstream dam 7 is 1.5 m. It carried out in.
[0024]
A comparison of the results is shown in FIG. Here, the inclusion index to the mold is the ratio of the number of inclusions flowing out to the mold through the nozzle for pouring molten steel into the mold with respect to the number of inclusions given at the inlet from the ladle. It is an index relative to the case as 1.
[0025]
As a result, if the position of the upstream dam 7 is 1.5 m using the apparatus of the present invention, the number index of inclusions in the mold is greatly reduced. Inclusion number index could be remarkably reduced by increasing the length to 0 m.
Further, if the position of the upstream weir 7 on the wake side is 2.5 m so as to satisfy the expression (1), inclusions with a diameter of 100 (μm) are not found, and inclusions flowing out into the mold can be eliminated. It was.
[0026]
Next, in the case where only the lower weir is not installed in the apparatus of the present invention, as a comparative example, the position of the molten steel outlet 4 to the mold when the upper weir 7 is at the position of L = 2.5 (m) The molten steel temperature was measured at a height of 0.2 m from the bottom of the tundish (described as the tundish outlet side). For reference, the molten steel temperature at the position of the injection nozzle 2 from the ladle (described as the tundish inlet side) was measured at a height of 0.2 m from the bottom of the tundish.
[0027]
As a result, as shown in FIG. 4, when the lower weir 6 is installed as in the apparatus of the present invention, the temperature is 5 (K) higher than when the lower weir 6 is not installed as in the comparative example. It was. Therefore, in the case where the lower weir 6 is present, it was confirmed that the heating temperature of the molten steel injected into the mold was raised and the substantial heat efficiency was increased.
[0028]
【The invention's effect】
As described above, the use of the apparatus of the present invention makes it possible to produce clean steel with good heating efficiency and few large inclusions.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a device configuration of the present invention.
FIG. 2 is an explanatory view schematically showing the flow generated by the apparatus of the present invention.
FIG. 3 is a comparison diagram of the distance between the upstream dam on the wake side of the embodiment and the mold position and inclusions.
FIG. 4 is a measurement comparison diagram of molten steel temperature on the tundish delivery side of the example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Ladle 2 Injection nozzle from ladle 3 Tundish 4 Molten steel outlet 5 to mold 5 Upper weir 6 Lower weir 7 Upper weir 8 Plasma 9 Electrode 10 Flow

Claims (1)

In the tundish arranged between the ladle for transporting molten steel from the refining process and the continuous casting mold, the nozzle position for injecting molten steel from the ladle to the tundish, and the nozzle for injecting molten steel from the tundish to the mold A pair of upper weirs and a lower weir placed between the pair of upper weirs, and a plasma heating electrode is disposed between the pair of upper weirs. The distance D (m) between the lower end of the steel and the bottom of the tundish, the depth H (m) of the molten steel at the portion, the average width W (m) of the molten steel channel, the throughput Q (t / min) of the molten steel, A tundish for continuous casting of steel having a heating function , characterized in that the distance L (m) between the upper weir and the molten steel outlet satisfies the following formula (1) .
L> α × { Q × (H + D / 2) ^1/2 } / (D × W) (1)
Where α: γ / { ρ × (β × g × ΔT) ^1/2 }
ρ: Molten steel density at the liquidus temperature of molten steel (kg / m ³ )
β: Body expansion coefficient of molten steel (1 / K)
g: Gravity acceleration
ΔT: Average molten steel temperature rise due to plasma heating (K)
γ: Proportional constant determined by operating conditions

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