JPH0685985B2 - Method for promoting floating separation of inclusions in molten steel in tundish - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for promoting the floating separation of inclusions in a tundish for continuous casting of steel.

[Conventional technology]

If molten steel after converter steel removal (primary refining) undergoes secondary refining such as vacuum degassing if necessary, when producing cast slabs with continuous casting equipment, the molten steel is longer nozzle than ladle (immersion nozzle). ) Or a sliding nozzle is poured into the tundish, while the inclusions are floated and separated in the tundish, while the immersion nozzle at the bottom of the tundish is poured into the mold (mold), and the inclusions are floated and cooled, and the slab is cast. Is pulled out as.

The floating separation of inclusions in the tundish is rarely performed in the ladle discharge flow affected region, but is performed in the extruded flow region.

However, recently, in order to improve production efficiency, high-speed casting is often performed by increasing the flow rate and flow rate of molten steel poured from the ladle, and as a result, the proportion of the discharge flow affected area increases and The floating separation property of the product tends to decrease.

As measures against this, (1) a method of reducing the influence of the discharge flow by providing a submerged weir and an overflow weir, as described in JP-A-51-138537, (2) a porous brick from the lower part of the tundish. A method of injecting an inert gas and stirring molten steel,
(3) A method in which a filter is provided between the tundish and the nozzle as in JP-A-58-176065, and (4) a tubular refractory material having openings is provided in the hot water receiving portion of the tundish and immediately above the immersion nozzle. There is known a method of adsorbing inclusions there.

[Problems to be solved by the invention]

However, in the method (1), there is a possibility that a stop zone (dead space area) of the flow of molten steel may be generated, which may adversely affect the floating separation property of inclusions. Is difficult to adjust, and there is a risk that inclusions that have floated up or flux that floats on the surface of the molten steel may become involved. Furthermore, in the method (3), it is necessary to replace the filter together with the clogging, which results in a high cost and is not practical, and in the method (4), a high adsorption effect cannot be expected.

Therefore, a main object of the present invention is to provide a method for promoting the floating separation separation of molten steel inclusions in a tundish that is high in the floating separation efficiency, does not decrease even after long-term operation, and is economical and practical.

[Means for solving problems]

The present invention for solving the above problems, when pouring molten steel into a tundish, divides the tundish into a ladle discharge flow affected region and an extrusion flow region by a weir, and in the extrusion flow region, the tundish Inner molten steel depth: H, molten steel volume in extruded flow area: V, molten steel flow rate per strand into the mold: Q, inclusion particles with a particle size of 50 μm according to Stokes' law are floated up to the molten steel surface in the tundish Velocity: Vy, Time for inclusion particles with a particle size of 50 μm to float to the surface of molten steel in the tundish: t ₅₀ , T d = Average residence time in tundish for inclusion particles of all particle sizes: T / t ₅₀ ≧ 0.8 …………………… (A) where T = V / Q …………………… (B) t ₅₀ = H / Vy ………… (C) It is characterized by pouring in.

In this case, when the distance between the center of the discharge flow from the ladle to the tundish and the surface of the discharge flow side of the weir is l, a weir having a large number of openings in the tundish is 0.5H ≦ l ≦ 2H. It is desirable that the tundish is divided into a ladle discharge flow influence region and an extrusion flow region.

[Action]

The particle size of inclusions in molten steel varies widely, and the amount is 50μ.
Although many of them have a size of m or less, it has been found that when a cast product is rolled into a product, a defect is a size of 50 μm or more.

Therefore, the present inventor paid attention to the inclusions having a size of 50 μm or more, and examined the floating separation property from various aspects.

As a result, it was found that it was effective to divide the tundish into a ladle discharge flow affected region and an extruded flow region by a weir, and to achieve levitation separation in the extruded flow region. In this case, in the form of the weir as in the method of (1) above, since a dead space region is generated, a multi-hole weir, especially 1
The porous weir with an opening area of 9 cm ² or more prevents stagnant molten steel flow and prevents blockage at the opening.
It was also found that it is preferable to divide each of the above regions.

As mentioned above, the presence of inclusions of 50 μm or more greatly affects the quality of the cast slab. Therefore, considering the floatability of inclusions of 50 μm or more, as will be described later, T / t ₅₀ (T; average retention time in tundish of inclusion particles of all particle sizes, t ₅₀ ; When the material particles float to the surface of the molten steel in the tundish) ≧ 0.8, the floating separation efficiency ζ [1-
(Number of inclusions after ascending) / Number of inclusions before ascending)] is obtained as desired. Here, T is represented as a ratio of the molten steel volume in the extrusion flow region (V) / the molten metal pouring flow rate (Q) into the mold per strand. Also t
₅₀ is expressed as the ratio of the floating velocity (Vy) to the molten steel surface in the tundish of inclusion particles having a particle diameter of 50 μm according to Stokes's molten steel depth (H) / Stokes' law.

Therefore, Vy can be obtained by Stokes' law, which will be described later.Therefore, by setting each of the fluctuation factors V, Q, and H such that T / t ₅₀ ≥ 0.8, the desired levitation separation efficiency ζ
Can be obtained.

On the other hand, if the position of the weir is determined as in the equation (D), the floating separation of inclusions can be effectively performed.

〔Example〕

 The present invention will be described in more detail below.

FIG. 1 shows a main part of a continuous casting facility from the viewpoint of the flow of molten steel. Molten steel in a ladle 1 is poured into a tundish 3 through a long nozzle (immersion nozzle) 2 or a sliding nozzle.

According to the present invention, the tundish 3 is provided with a porous weir 4 having many openings 4a, 4a ... As shown in FIG. 2, and is divided into a ladle discharge flow influence region Z and an extrusion flow region Y. There is. The molten steel poured into the tundish 3 flows from the ladle discharge flow affected region Z to the extruded flow region Y through the opening a of the porous weir 4, and in the extruded flow region Y, inclusions are floated and the flux 5 Is adsorbed on.
The molten steel from which the inclusions have been removed is poured into the mold 7 from the immersion nozzle 6 provided in the lower part of the end of the tundish 3.

In FIG. 1, a porous weir 4'is shown on the left side of the long nozzle 2, but this is for another strand.

By the way, the floating separation efficiency ζ of inclusions is defined by the equation (1).

This equation (1) can be rewritten by using the time t for the inclusion particles of all particle sizes to rise to the surface of the molten steel in the tundish: t and the average residence time of the inclusion particles for all particle sizes in the tundish: T. , C ^OUT = C ^IN · exp (−T / t), ζ = 1−exp (−T / t) ………… (2) Considering particles with a particle size of 50 μm or more, While deriving this equation (3), a water model experiment was conducted to change L (distance from the extrusion flow region side surface of the porous weir 4 to the center of the immersion nozzle 6), H and dp (particle diameter) while
When the relationship with T / t (-) was examined, it was shown in Figs.
The relationship shown in the figure was obtained, and it was found that equation (3) is indeed valid.

As is clear from both figures, when T / t ₅₀ ≧ 0.8, ζ becomes approximately 0.58 or more, and sufficient separation efficiency can be obtained.

Therefore, conversely, in order to satisfy the condition of T / t ₅₀ ≧ 0.8, since T = V / Q and t ₅₀ = H / Vy, respectively, V (molten steel melt in the extrusion flow region; L × H × W (width)), Q (flow rate of molten steel poured into the mold per strand), H
It suffices to set the condition (the depth of molten steel in the tundish). In addition, Vy is calculated by the following equation (4) assuming that it floats according to Stokes' law.

Vy = {g (ρ _P −ρ) / 18μ} ・ d _P ² ……… (4) where g: Gravitational acceleration (= 9.8 m ² / s) ρ _P : Density of inclusions (≈2200 Kg / m) ³ However, Al ₂ O-C
Estimated value of aO inclusions ρ _P : Density of molten steel (≈7000 Kg / m ³ ) μ: Viscosity of molten steel (≈0.006 Pa · s) d _P : Diameter of inclusion particles (m) The installation is also an important factor, and it is desirable that the size l shown in FIG. 1 be within the range of the following formula (D) in relation to H.

0.5H ≦ l ≦ 2H (D) The perforated weir 4 is effective for rectifying the ladle discharge flow and increasing the extrusion flow region Y. If l is less than 0.5H, melting damage of the porous weir 4 due to the ladle discharge flow cannot be avoided,
Instead, it promotes the new generation of inclusions. On the other hand, when it exceeds 2H, the extruded flow region becomes small and it becomes impossible to sufficiently perform the floating separation. Therefore, the maximum value of l is l
Exists in the vicinity of = H.

If the opening area of the porous weir 4 is too small, the effect of preventing the stagnation of the melt flow will be reduced, and in order to prevent the opening from being blocked by the molten steel, it is 9 cm ² (opening diameter about 3.4 cm) or more, especially 12 cm ² (Opening diameter about 3.9 cm) or more is preferable. However, if it is 20 cm ² (opening diameter of about 5 cm) or more, the dividing effect as a weir is reduced. It is desirable that the number of openings is at least one per 50 cm ^{2 of} weir surface area so that dead spaces are not generated. The shape of the opening is circular,
A prism, an ellipse, etc. can be used. However, something close to a circle is good. The material of the porous weir 4 is a refractory material. If necessary, the porous weirs 4 may be arranged at intervals in the flow direction of the molten steel.

(Example) Next, the effect of the present invention will be clarified by an experimental example.

As shown in Table 1, various types of tundish were manufactured and subjected to actual machine tests. The floating separation effect shown in the table was obtained.

The molten steel flow rate Q per strand is 2.94 ton / min.
= 7 × 10 ^-3 m ³ / sec. The amount of inclusions is 10 kg of slab.
The number of inclusions of slime-extracted inclusions (particle size: 50 to 200 μm) is shown. Further, the floating speed Vy of the inclusions of 50 μm is approximately 0.11 cm / sec when calculated by the equation (4).

The shape generally used in the past is No. 2, but it can be seen that the amount of inclusions decreases when T / t ₅₀ is increased according to the present invention.

〔The invention's effect〕

As described above, according to the present invention, the floating separation efficiency of inclusions is high, stable operation is possible without blockage due to molten steel, and high floating separation efficiency is obtained mainly by selecting the tundish shape, which is economical.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a tundish portion in a continuous casting facility, FIG. 2 is an example of an opening shape of a porous weir and its arrangement, and FIGS. 3 and 4 are relational diagrams of results of water model experiments. 1 ... Ladle, 3 ... Tundish, 4 ... Porous weir, 4a ... Opening, 7 ... Mold

Claims (2)

[Claims] 1. When pouring molten steel into a tundish,
The tundish is divided into a ladle discharge flow affected region and an extruded flow region by a weir, and in the extruded flow region, the molten steel depth in the tundish is H, the molten steel volume in the extruded flow region is V, and the molten steel per strand is Pouring flow rate into mold: Q, Floating speed of inclusion particles with particle size 50 μm to molten steel surface in tundish according to Stokes' law: Vy, Time for inclusion particles with particle size 50 μm to float to molten steel surface in tundish: t ₅₀ , the average residence time in the tundish of inclusion particles of all particle sizes: T, T / t ₅₀ ≧ 0.8 ……………… (A) where T = V / Q …… ………………… (B) t ₅₀ = H / Vy ………… (C) A method for promoting the floating separation of inclusions in molten steel in the tundish, which is characterized by pouring the molten metal under the conditions. 2. When the distance between the center of the discharge flow from the ladle to the tundish and the surface of the weir on the discharge flow side is l, the weir having a large number of openings in the tundish is 0.5H ≦ l ≦ 2H. The method for promoting floating separation of inclusions in molten steel in a tundish according to claim 1, wherein the tundish is placed at a position D) and the tundish is divided into a ladle discharge flow influence region and an extrusion flow region.