JPH11320045A - Nozzle for continuous casting and method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimally control the fluid of molten steel in a mold always by positioning a gas suction hole on a half surface so as to be the reverse side to the opening part of a sliding nozzle and also, at a height satisfying a specific condition, at the time of disposing a discharging hole so as to face to the side of the short side of a mold. SOLUTION: A sliding plate 9 of a sliding nozzle 8 is slidingly moved from the right side to the left side, then, the passing way of molten steel 10 in a tundish nozzle is drawn to execute the adjustment of a flow rate, the molten steel 10 flows through the opening hole part 7 side of the sliding nozzle and a gas storing part 11 is formed on the opposite side. In this way, the sticking of the molten steel to the gas suction hole 2 can be prevented by securing H1 /H2 >=2.0 on the side opposite to the opening hole part 7 of the sliding nozzle in the ordinary operation. Wherein, H1 is the distance (m) from the upper end of the tundish nozzle 1 to the molten steel surface 13 in the mold and H2 is the distance (m) from the gas suction hole 2 at the lowermost part to the molten steel surface 13 in the mold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tundish nozzle for continuous casting of steel and a method for continuous casting of steel using the tundish nozzle for continuous casting.

[0002]

2. Description of the Related Art Normally, molten steel is injected from a tundish into a mold through a refractory tundish nozzle having an inverted Y-shaped discharge hole provided in the tundish. Molten steel flowing into the mold from the left and right discharge holes of the tundish nozzle collides with the short piece of the mold, and is then split vertically, one of which flows downward along the short piece,
The other rises and becomes a molten steel surface flow. If the molten steel surface flow is too strong, powder entrainment occurs on the molten steel surface, and if the molten steel surface flow is too weak, heat supply to the molten steel surface is insufficient and the partially solidified deckle is formed in the mold. Be brought to Further, when the short piece descending flow is too strong, the penetration depth of the molten steel toward the lower part in the mold becomes deep, so that the inclusions in the molten steel cannot be completely lifted and are trapped inside the slab. For this reason, it is desired that the tundish nozzle be given a surface flow velocity in a range that does not cause powder entrainment and deckle on the molten steel surface, and furthermore, make the penetration depth of the molten steel as small as possible. Using a tundish nozzle having a shape, for example, a tundish nozzle disclosed in Japanese Patent Application Laid-Open No. 61-14051, the average flow of molten steel in the mold is controlled to an approximately appropriate range.

[0003]

However, the flow rates of the molten steel flowing out of the left and right discharge holes of the tundish nozzle are not always the same, and there is a bias in the left and right discharge flows due to the turbulence of the molten steel flow in the tundish nozzle. Arises
This bias changes over time. When such a drift phenomenon occurs, the surface velocity of the molten steel and the penetration depth of the molten steel simultaneously increase on the side of the higher discharge velocity, so that surface defects due to powder and internal defects due to alumina occur frequently, and the quality of the slab is high. Is significantly reduced. In other words, only the average flow control in the mold by the conventional tundish nozzle shape prevents the drift phenomenon caused by the turbulence of the molten steel flow in the tundish nozzle, and always controls the molten steel flow in the mold to an appropriate range. I can't.

The present invention solves these problems in the conventional tundish nozzle. By rectifying the flow of molten steel in the tundish nozzle, it is possible to always control the flow of molten steel in the mold optimally. It is an object to provide a tundish nozzle for continuous casting and a continuous casting method using the same.

[0005]

According to the present invention, there is provided (1) a tundish nozzle for continuous casting of steel, wherein one or a plurality of gas suction holes are provided on an inner wall surface of the tundish nozzle and a gas suction hole is provided on a back surface thereof. And a gas suction structure in which an exhaust fitting for connecting a gas suction pipe is arranged in the closed chamber, and a tundish such that the discharge hole faces the short side of the mold. When the nozzle is installed, the gas suction hole is located on the half surface opposite to the sliding nozzle opening, and is at a height that satisfies the following conditions: A tundish nozzle for continuous casting . H ₁ / H ₂ ≧ 2.0 where H ₁ is the distance (m) from the upper end of the tundish nozzle to the molten steel surface in the mold, and H ₂ is the distance from the gas suction hole at the lowest end to the molten steel surface in the mold ( m).

(2) The tundish nozzle according to (1), wherein an inert gas blowing hole is provided separately from the gas suction hole. (3) The tundish nozzle according to (1) or (2) above, so that the discharge hole faces the short side of the mold, and the position of the gas suction hole is on the side opposite to the opening of the sliding nozzle. A continuous casting method of steel, wherein gas is sucked from gas suction holes arranged in the tundish nozzle, and casting is performed while reducing the pressure in the tundish nozzle. (4) Using the tundish nozzle described in (1) or (2) above, gas is sucked from a gas suction hole arranged in the tundish nozzle, and the pressure in the tundish nozzle satisfies the following conditions. A continuous casting method for steel, characterized in that casting is performed while controlling the casting. H ₃ ≦ H ₂ and H ₃ /D≧2.5 where H ₃ is the height of molten steel surface in the tundish nozzle
(m), D is the inside diameter (m) of the tundish nozzle. (5) Using the tundish nozzle described in (1) or (2) above, gas is suctioned from the gas suction holes arranged in the tundish nozzle, and the pressure in the tundish nozzle satisfies the following conditions. A continuous casting method for steel, characterized in that casting is performed while controlling the casting. _{P N ≦ P A -2.5 · D} · ρ · g and _{P N ≧ P A -H 2 ·} ρ · g where, P _N is the pressure in the tundish nozzle (Pa), P _A atmospheric pressure (Pa) , H ₂ is the distance from the gas suction hole at the bottom end to the surface of molten steel in the mold (m), D is the inside diameter of the tundish nozzle (m), ρ is the density of molten steel (kg / m ³ ), and g is the gravitational acceleration (m
/ s ² ). It is.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS. Generally, an inert gas is blown into the tundish nozzle 1 in order to prevent the tundish nozzle 1 from being blocked. In this case, the tundish nozzle 1 made of an alumina graphite material is hard to wet with molten steel, and is located between the inner wall of the tundish nozzle and the molten steel 10 below the position of the sliding plate 9 of the sliding nozzle 8 for controlling the injection flow rate. A gas reservoir 11 is formed, and the molten steel falls freely in the tundish nozzle. The free falling flow of the molten steel is not restrained by the inner wall of the tundish nozzle, and is unstable and easily disturbed, and the disturbance is transmitted to the molten steel surface 12 in the tundish nozzle, thereby causing a drift phenomenon. In addition, the inert gas blown into the tundish nozzle is a gas reservoir once formed between the inner wall of the tundish nozzle and the molten steel 10.
The pressure in the tundish nozzle changes periodically because it stays at 11 and is then intermittently blown into the mold. The periodic pressure change causes the molten steel surface 12 in the tundish nozzle to vibrate up and down, further promoting the drift phenomenon.

The above-described drift generation mechanism has been clarified by the present inventors through a model experiment and analysis of a continuous casting machine using water or mercury. Based on this finding, the turbulence of molten steel flow in a tundish nozzle was found. In order to prevent the drift phenomenon caused by turbidity and to optimally control the flow of molten steel in the mold, the free-fall flow in the tundish nozzle caused by the injection of the inert gas should be close to a stable full flow. Deemed important.

With reference to the molten steel surface 13 in the mold, the height of the molten steel surface 12 in the tundish nozzle (indicated by H ₃ (m) in FIG. 2) is expressed by the following equation (1).

[0010] _{H 3 = (P A -P N} ) / (ρ · g) (1) where, P _N is the pressure in the tundish nozzle (Pa), P _A atmospheric pressure (Pa), ρ is the molten steel Density (kg / m ³ ), g is gravitational acceleration (m
/ s ² ). All pressures are absolute pressures. That is, by lowering the pressure in the tundish nozzle, the molten steel surface height 12 in the tundish nozzle can be increased, and theoretically, when the pressure in the tundish nozzle is reduced to 0 Pa, the molten steel surface height in the tundish nozzle can be increased. the height H ₃ 1.
5m rise. In general, the distance from the top of the tundish nozzle (bottom of the tundish) to the molten steel surface 13 in the mold is about 1 m. Therefore, even if free fall flow occurs due to the blowing of inert gas, the pressure inside the tundish nozzle is reduced. By lowering it, the molten steel surface height 12 in the tundish nozzle can be maintained high, and the drift phenomenon can be prevented.

The pressure inside the tundish nozzle is controlled by sucking an inert gas from the inside of the tundish nozzle.
When 10 comes in contact, it will be sucked up to molten steel. In addition, even when a refractory that does not penetrate the molten steel is used, if the molten steel comes into contact, the suction capacity is reduced. For this reason, it is effective to provide a gas suction hole 2 in a portion of the inner wall of the tundish nozzle where the molten steel 10 does not contact, and to suck an inert gas through this hole. When performing gas suction by this method, it is essential to arrange the gas suction holes 2 at positions where molten steel does not come into contact.

Therefore, a tundish nozzle used for ordinary casting was recovered after casting, and the contact position of molten steel was evaluated. FIG. 3 shows the relationship between the position in the tundish nozzle and the thickness of the molten steel at that time. The height position is set upward starting from the molten steel surface 13 in the mold. Here, a portion that opens when the sliding nozzle is half-opened is referred to as a sliding nozzle opening. As a result, on the opening side of the sliding nozzle, molten steel was observed to adhere from the molten steel surface 13 in the mold to the upper end of the tundish nozzle, whereas on the opposite side of the opening side of the sliding nozzle, the molten steel surface 13 No molten steel was observed in the upper half up to the top of the tundish nozzle. this is,
As the sliding plate 9 of the sliding nozzle 8 slides from the right side to the left side as shown in FIG. 2, the path of the molten steel is narrowed and the flow rate of the molten steel is adjusted. It is considered that this is because a gas reservoir 11 is formed on the side opposite to the sliding nozzle opening side while flowing on the part 7 side. As shown in the experimental results in FIG. 3, in the normal operation, H ₁ and H ₂ shown in FIG.
For, it is possible to prevent the molten steel from adhering to the gas suction holes when securing the H ₁ / H ₂ ≧ 2.0.

FIG. 4 shows how the molten steel adheres in the circumferential direction at the center from the molten steel surface 13 in the mold to the upper end of the tundish nozzle. According to this, molten steel was found to adhere to one half of the circumferential direction on the sliding nozzle opening 7 side, but no molten steel was found to adhere to the other half. Therefore, if the gas suction hole 2 is arranged on the half surface of the tundish nozzle opposite to the sliding nozzle opening side, the adhesion of molten steel to the gas suction hole 2 can be prevented.

On the other hand, when the pressure in the tundish nozzle is reduced, the molten steel surface height 12 in the tundish nozzle also increases accordingly. Even in such a case, in order to prevent the molten steel from contacting the gas suction holes 2, it is necessary to control the pressure of H ₂ and H ₃ shown in FIG. 2 while ensuring that H ₃ ≦ H ₂ .

In order to quantitatively evaluate the relationship between the filling state in the tundish nozzle (the molten steel surface height in the tundish nozzle 12) and the drift phenomenon, an experiment was performed using a mercury model device of a continuous casting machine. The tundish nozzle was made of transparent acrylic and its shape was basically 1 / 2.5 size of the tundish nozzle used in the gas suction experiment, but the inner diameter of the tundish nozzle was changed variously. The mold size is 100mm thick and 1000mm wide. Mercury
While supplying the mold through the tundish nozzle at a flow rate of 20 l / min, the discharge flow rates on the left and right sides of the tundish nozzle were measured with a propeller velocimeter. At the same time, the mercury surface height 12 in the tundish nozzle was evaluated by visual observation. Ar gas was injected at a flow rate of 1.2 Nl / min from the gas injection refractory 5 provided in the tundish nozzle. The drift phenomenon was evaluated by a value obtained by dividing the absolute value of the difference between the left and right discharge flow velocities at the tundish nozzle by the average value of the left and right discharge velocities and multiplying by 100 (a drift index). Figure 5 shows the mercury surface height H ₃ / in the tundish nozzle.
The relationship between the inner diameter D of the tundish nozzle and the drift index is shown.
It is understood that the drift phenomenon can be suppressed by setting H ₃ / D to 2.5 or more. This is considered to be because the filling area is expanded by securing H ₃ / D of 2.5 or more, and the turbulence of the free-fall flow is reduced to some extent by the tundish discharge hole 6.

In the actual casting of molten steel, the surface height H ₃ of the molten steel in the tundish nozzle cannot be detected. Therefore, in a model experiment of mercury or the like that is difficult to get wet with the tundish nozzle, gas is injected under the same inert gas blowing conditions as the actual machine. The relationship between the pressure in the closed chamber 3 arranged behind the suction hole 2 and the liquid surface height H ₃ in the tundish nozzle is quantified in advance, and the relationship between H ₃ /D≧2.5 and H ₃ ≦ H ₂ is determined. The pressure in the closed chamber 3 may be controlled so as to satisfy the conditions. The pressure in the closed chamber 3 of the tundish nozzle can be measured by attaching a pressure gauge to a pipe connected to the closed chamber.

Further, when the drift suppression condition and the condition in which the molten steel 10 does not contact the gas suction hole 2 are indicated by the pressure P _N (Pa) in the tundish nozzle using the equation (1), P _N ≦ P _A −
2.5 · D · ρ · g, since _{P N ≧ P A -H 2 ·} ρ · g is obtained, detects the pressure in the tundish nozzle directly, the pressure in the tundish nozzle so as to satisfy the above conditions Pressure reduction control is also possible. Furthermore, if the difference between the pressure in the tundish nozzle and the pressure in the closed chamber or the exhaust mouthpiece is measured in advance as a relationship with the gas suction amount,
During casting, without measuring the pressure in the tundish nozzle, measure the pressure in the closed chamber or the exhaust cap,
Control can also be performed using the pressure in the tundish nozzle estimated from the pressure.

[0018]

The present invention will be described below with reference to examples and comparative examples. The present inventors assumed that the inner diameter was 90 mm, the length was 1200 mm, the discharge hole diameter was 90 mm, and the discharge hole angle was 35 ° as shown in FIG.
The inner wall of the alumina graphite tundish nozzle 1 has a length of 200 mm to prevent the tundish nozzle from clogging.
Inert gas refractory 5 made of alumina graphite with a thickness of 10 mm, 200 mm from the top of the tundish nozzle,
At a position of 300mm, 6 pieces each on the half surface in the circumferential direction, a total of 1
Using a tundish nozzle 1 with two gas suction holes 2 with an inner diameter of 0.5 mm, a slab size of 250 mm (thickness)
1250t of ultra low carbon steel with × 1830mm (width) and carbon concentration of 30ppm was cast at a casting speed of 1.8m / min. Ar gas was injected at 10 Nl / min from the gas injection refractory 5 above the tundish nozzle to prevent blockage of the tundish nozzle. The continuous casting machine used for the test had two strands. One of the strands had the gas suction hole 2 positioned at the sliding nozzle opening 7 side, and the other had the gas suction hole 2 positioned at the sliding nozzle opening. A tundish nozzle was installed on the opposite side to the 7 side. Gas suction was performed on both strands using a vacuum pump having a pressure control function so that the pressure in the closed chamber 5 of the tundish nozzle was 100 Pa. Then, although the strand where the tundish nozzle 1 was installed was completely cast without any problem so that the position of the gas suction hole 2 was on the opposite side to the sliding nozzle opening 7 side, the position of the gas suction hole 2 was changed to the sliding nozzle opening.
In the strand installed on the 7th side, the molten steel was sucked immediately after the casting of the molten steel started, and the casting was stopped.

Subsequently, a tundish nozzle was installed so that the position of the gas suction hole 2 was on the opposite side of the sliding nozzle opening 7 for both of the two strands, and the same casting as described above was performed. At this time, gas suction was performed using a vacuum pump having a pressure control function such that the pressure in the closed chamber 3 communicating with the gas suction hole 2 of the tundish nozzle on one of the strands was 100 Pa. The cast slab was cut into 8500 mm lengths to make one coil unit. This slab is hot-rolled and cold-rolled by a conventional method.
A cold-rolled steel sheet with a coil of 0.7 mm × width of 1830 mm was used. The slab quality was visually observed on the inspection line after cold rolling.
The number of surface defects generated per coil was evaluated. In addition, the occurrence of drift was evaluated by detecting the difference in the molten steel surface height of both short pieces from the thermocouple embedded in the mold.

In the embodiment in which the pressure inside the tundish nozzle was controlled to be reduced, the difference between the surface heights of the molten steel of the two short pieces was 5 mm or less, and no drift occurred, so no slab defects were generated. No molten steel was found near the gas suction hole 2. In contrast, in the comparative example in which the pressure inside the tundish nozzle was not controlled, the difference in the molten steel surface height of both short pieces reached a maximum of 40 mm, and the drift was severe, so that the powder was caught and the alumina inclusions were caught. Surface defects and internal defects occurred.

[0021]

As described above, since the drift phenomenon can be prevented by the present invention, the flow of molten steel in the mold is optimally controlled, and the quality of the slab is remarkably improved. In addition, stable control is possible without molten steel adhering to the gas suction holes for controlling the pressure.

[Brief description of the drawings]

FIG. 1 is a view showing a tundish nozzle used for casting.

FIG. 2 is a diagram showing a state of a flow in a tundish nozzle.

FIG. 3 is a diagram showing a relationship between a position in a height direction in a tundish nozzle and an adhesion thickness of molten steel.

FIG. 4 is a diagram showing a relationship between a circumferential position in a tundish nozzle and an adhesion thickness of molten steel.

FIG. 5 is a diagram showing the relationship between the molten steel surface height in the tundish nozzle / the tundish nozzle inner diameter and the drift index.

[Explanation of symbols]

 1: Tundish nozzle 2: Gas suction hole 3: Tundish nozzle closed chamber 4: Exhaust fitting 5: Refractory for blowing inert gas 6: Discharge hole 7: Sliding nozzle opening 8: Sliding nozzle 9: Sliding plate 10: molten steel in the tundish nozzle 11: gas reservoir 12: molten steel surface in the tundish nozzle 13: molten steel surface in the mold 14: mold (short side) 15: molten steel attached to the tundish nozzle

Claims (5)

[Claims] 1. A tundish nozzle for continuous casting of steel, wherein one or more gas suction holes are provided on the inner wall surface of the tundish nozzle, and a closed chamber communicating with the gas suction holes is provided on the back surface thereof. It has a gas suction structure with an exhaust fitting for connecting a gas suction pipe to the chamber.
When the tundish nozzle is installed so that the discharge hole faces the short side of the mold, the gas suction hole is located on the half surface opposite to the sliding nozzle opening, and has a height that satisfies the following conditions. A tundish nozzle for continuous casting, which is located at a position. H ₁ / H ₂ ≧ 2.0 where H ₁ is the distance (m) from the upper end of the tundish nozzle to the molten steel surface in the mold, and H ₂ is the distance from the gas suction hole at the lowest end to the molten steel surface in the mold ( m). 2. The tundish nozzle according to claim 1, wherein an inert gas blowing hole is provided separately from the gas suction hole. 3. The tundish nozzle according to claim 1 or 2, wherein the discharge hole faces the short side of the mold, and the position of the gas suction hole is on the side opposite to the sliding nozzle opening. A continuous casting method of steel, wherein gas is sucked from gas suction holes arranged in the tundish nozzle, and casting is performed while reducing the pressure in the tundish nozzle. 4. Using the tundish nozzle according to claim 1 or 2, gas is suctioned from gas suction holes arranged in the tundish nozzle, and the pressure in the tundish nozzle satisfies the following conditions. A continuous casting method for steel, characterized in that casting is performed while controlling the casting. H ₃ ≦ H ₂ and H ₃ /D≧2.5 where H ₃ is the height of molten steel surface in the tundish nozzle
(m) and D are the inner diameter (m) of the tundish nozzle. 5. Using the tundish nozzle according to claim 1 or 2, gas is suctioned from a gas suction hole arranged in the tundish nozzle, and the pressure in the tundish nozzle satisfies the following conditions. A continuous casting method for steel, characterized in that casting is performed while controlling the casting. _{P N ≦ P A -2.5 · D} · ρ · g and _{P N ≧ P A -H 2 ·} ρ · g where, P _N is the pressure in the tundish nozzle (Pa), P _A atmospheric pressure (Pa) , H ₂ is the distance (m) from the gas suction hole at the bottom end to the surface of molten steel in the mold, D is the inside diameter of the tundish nozzle (m), ρ is the density of molten steel (kg / m ³ ), and g is the gravitational acceleration (m
/ s ² ).