JP2012020294A - Method for changing immersion depth of immersion nozzle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make molten metal surface fluctuation of molten steel inside a mold hard to occur in the early stages of casting and in the last stages of the casting.SOLUTION: In continuous casting, an immersion depth of an immersion nozzle into the molten steel inside the mold is deepened until a casting speed reaches a constant speed after a casting start. The immersion depth of the immersion nozzle when the casting speed reaches the constant speed of the continuous casting after the casting start is shallower than the immersion depth of the immersion nozzle until the casting speed reaches the constant speed after the casting start. Thereafter, the immersion depth of the immersion nozzle is gradually deepened when the casting speed is constant and until a state which is a state close to a casting end. The immersion depth of the immersion nozzle when the casting speed reaches the constant speed of the continuous casting after the casting start is set to be shallowest out of the immersion depth during the continuous casting.

Description

  The present invention relates to a method for changing the immersion depth of molten nozzle in a mold of an immersion nozzle in continuous casting.

  In continuous casting of steel, molten steel is injected into a mold of a continuous casting machine from a tundish by an immersion nozzle immersed in the mold. At this time, the immersion nozzle is particularly easily melted by the slag present on the molten steel in the mold. Therefore, in order to prevent breakage of the immersion nozzle, usually, the melting damage of the immersion nozzle at the same location is prevented by changing the immersion depth of the immersion nozzle into the molten steel in the mold. Among them, in the method disclosed in Patent Document 1, when the molten steel of the first charge is injected into the mold, the immersion depth is set to the shallowest position, and the molten steel after the second charge is injected into the mold. Sometimes, the immersion depth is once set to the deepest position, and then the immersion depth is changed stepwise to the shallowest position.

JP 2004-202538 A

  In the method disclosed in Patent Document 1, the immersion depth of the immersion nozzle is set at the shallowest position in the initial stage of casting. Further, in this method, at the time of casting after the second charge, the immersion depth of the immersion nozzle is once set to the deepest position, and then the immersion depth of the immersion nozzle is changed to the shallowest position. At the end, the immersion depth is set at the shallowest position. Therefore, in the continuous casting method disclosed in Patent Document 1, the immersion depth is set at the shallowest position in the initial casting stage and the final casting stage. When the immersion depth is shallow, the discharge hole of the immersion nozzle is located near the molten steel surface.

  By the way, at the initial stage of casting, molten steel is injected into an empty mold, and drawing of the slab is started in the mold, so that the molten steel surface fluctuation in the mold is large. In addition, as the immersion nozzle is used, deposits such as aluminum oxide adhere to the inner surface of the immersion nozzle, so that at the end of casting, the inner diameter of the immersion nozzle becomes smaller or the substantial diameter of the discharge hole becomes smaller. As a result, the molten steel flow injected into the mold becomes faster, and the molten metal level fluctuation of the molten steel in the mold becomes larger.

  If the discharge hole is located near the molten metal surface when the molten steel surface fluctuation in the mold is large, the discharge hole formed at the bottom of the immersion nozzle will be on the molten metal surface when the molten steel surface is significantly lowered. It may appear and the discharge hole may be exposed to the atmosphere. In this case, since the molten steel flowing out from the discharge hole comes into contact with the atmosphere and the molten steel is oxidized, the quality of the slab (product) is deteriorated.

  Also, if the immersion nozzle immersion depth is set to the shallowest position at the end of casting, the immersion nozzle discharge hole and the molten metal surface are close to each other. As a result, the hot water surface is likely to fluctuate greatly. Therefore, mold powder on the molten metal surface is caught in the molten steel, and surface defects such as shavings are generated in the product (a product obtained after the cast slab is rolled), resulting in a reduction in product quality.

  In the method of Patent Document 1, since the immersion depth of the immersion nozzle is set at the shallowest position at the initial stage of casting when the molten metal surface fluctuation is large and at the end of casting when the molten steel flow is fast, the above-described problem is likely to occur. it is conceivable that. Therefore, the quality of the product is deteriorated in the early casting stage and the late casting stage.

  Then, an object of this invention is to provide the immersion nozzle immersion depth change method in the continuous casting which can manufacture a high quality product in the casting initial stage and the casting final stage.

  The immersion nozzle immersion depth changing method in the continuous casting of the present invention is a method in which molten steel is poured into a mold through an immersion nozzle and continuously cast until the casting speed reaches a constant speed after the start of casting. The immersion depth of the immersion nozzle into the molten steel in the mold is increased. Thereafter, the immersion depth of the immersion nozzle when the casting speed reaches a constant speed after the start of casting is made shallower than the immersion depth of the immersion nozzle until the casting speed reaches the constant speed after the start of casting. Then, the immersion depth of the immersion nozzle is gradually increased from the time when the casting speed reaches a constant speed after the start of casting until the casting speed is constant and close to the end of casting. Here, “constant casting speed” means a casting speed in a steady state of continuous casting. In addition, since the start of casting indicates the time when injection of molten steel into the mold is started, the start of casting indicates immediately after the start of injection of molten steel into the mold.

According to the present invention, since the immersion depth of the immersion nozzle into the molten steel in the mold is changed stepwise during continuous casting, the immersion nozzle slag line (contact with the slag existing on the immersion nozzle molten steel). Part) can be changed. Therefore, it is possible to prevent the immersion nozzle from being melted at the same location, so that the life of the immersion nozzle can be extended.
In the present invention, the immersion depth of the immersion nozzle in the molten steel in the mold is set deep at the initial stage of casting when the molten steel surface fluctuation in the mold is large and at the end of casting when the molten steel flow is fast. Therefore, even if the molten metal surface fluctuation is large, the discharge holes formed in the lower part of the immersion nozzle are not easily exposed to the atmosphere, and thus the oxidation of the molten steel injected from the immersion nozzle can be prevented. Moreover, since the immersion depth is set deep when the molten steel flow becomes faster, the molten metal surface is not easily affected by the molten steel flow flowing from the discharge holes. Therefore, it is possible to prevent the hot water level fluctuation from increasing. Therefore, a high-quality product can be manufactured at the initial stage of casting when the molten steel surface fluctuation of the molten steel in the mold is large and at the end of casting when the molten steel flow is fast.

  According to the method of changing the immersion depth of the immersion nozzle in the continuous casting of the present invention, the immersion nozzle slag line in the molten steel in the mold of the immersion nozzle is changed stepwise during the continuous casting. Can be changed. Therefore, it is possible to prevent the immersion nozzle from being melted at the same location, so that the life of the immersion nozzle can be extended. Also, since the immersion depth of the immersion nozzle in the molten steel in the mold is set deep at the beginning of casting, when the molten steel surface fluctuation in the mold is large, and at the end of casting when the molten steel flow is fast, injection from the immersion nozzle is performed. The oxidation of molten steel can be prevented. Therefore, high quality slabs can be manufactured at the initial casting stage and the final casting stage.

It is a figure which shows the structure of a part of continuous casting machine. It is a figure which shows immersion depth and casting speed. (A) And (b) is a figure which shows the structure of a part of immersion nozzle. (A) And (b) is a figure which shows the structure of a part of immersion nozzle. It is a figure which shows an example of the experimental conditions of an Example.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  In FIG. 1, the structure of the tundish 1, the immersion nozzle 2, and the casting_mold | template 3 which are some continuous casting machines is shown. The tundish 1 holds molten steel supplied from a ladle (not shown). Molten steel in the tundish 1 is injected into the mold 3 through an immersion nozzle 2 connected to the tundish 1. In the mold 3, the molten steel 5 is cooled and pulled out downstream of the continuous casting machine while forming a solidified shell 5 a, and then completely solidified into a cast piece. The continuous casting method of this embodiment can be applied to all castings of slabs, blooms and billets. Moreover, the continuous casting method of this embodiment is applicable to various continuous casting machines, such as a vertical bending type continuous casting machine and a curved bending type continuous casting machine.

(Immersion nozzle)
As shown in FIG. 1, the immersion nozzle 2 is provided vertically in the mold 3. In the present embodiment, FIG. 1 shows a two-hole immersion nozzle 2 having a bottomed cylindrical shape and having a pair of opposed discharge holes 2 a and 2 b slightly above the inner bottom of the immersion nozzle 2. It is shown. The discharge holes 2a and 2b are opposed to a pair of inner surfaces 3a and 3b, respectively, of the mold 3 having a rectangular cross section. When casting is started and the molten steel is stored in the mold 3 to some extent, the lower part of the immersion nozzle 2 is immersed in the molten steel 5 in the mold 3. Then, the discharge holes 2 a and 2 b are arranged in the molten steel 5 in the mold 3. The molten steel flow from the discharge holes 2a and 2b first proceeds obliquely downward (dotted line in FIG. 1), collides with a pair of inner surfaces 3a and 3b facing each other of the mold 3, and branches in the vertical direction. And the upward flow Q and the downward flow R of molten steel are formed.

  Although not shown in FIG. 1, a valve capable of adjusting the amount of molten steel passing through the inside of the immersion nozzle is provided above the immersion nozzle 2. By opening and closing this valve, the amount of molten steel injected into the mold 3 can be adjusted.

  In the mold 3, a molten powder (slag) 6 and a powder powder 7 float in order on the molten metal surface of the molten steel 5. By providing the molten powder 6 and the powder powder 7 on the molten steel 5, it is possible to prevent the molten steel 5 from being exposed to the atmosphere and being oxidized. In addition, since the component contained in molten powder 6 reacts with the component contained in immersion nozzle 2, the part (slag line) which contacted molten powder 6 of immersion nozzle 2 is especially easy to melt.

  Next, one of the objects of the present invention is to change the slag line of the immersion nozzle (the portion where the immersion nozzle contacts the slag on the surface of the molten steel). A method for changing the immersion depth will be described.

(Immersion depth of immersion nozzle and casting speed)
FIG. 2 shows an example of a method for changing the immersion depth of the immersion nozzle and an example of a change in casting speed. 2 indicates the relationship between the immersion depth of the immersion nozzle and time, and the alternate long and short dash line in FIG. 2 indicates the relationship between casting speed and time.

  The immersion depth (Y axis in FIG. 2) is the length of the immersion nozzle immersed in the molten steel in the mold, and the distance from the molten steel surface in the mold to the lower end (outer bottom) of the immersion nozzle (see FIG. 2). D) shown in FIG. The time (X axis in FIG. 2) is set to 0 when the slab in the mold starts to be drawn downward (when the slab in the mold starts to be drawn downward) (in FIG. Start ”). The casting speed (Y axis in FIG. 2) is small immediately after the start of casting, and is stable after continuous casting is in a steady state. After that, the casting speed is decreasing just before the end of continuous casting. Note that immediately after the start of casting, sufficient molten steel is not secured in the mold, so the casting speed is low. Moreover, in FIG. 2, when continuous casting is in a steady state, the casting speed is constant. In addition, since the time of a casting start shows the time when injection | pouring of the molten steel into a casting_mold | template is started, immediately after the above-mentioned casting start shows the immediately after injection | pouring of the molten steel into a casting_mold | template.

(Dipping depth at the beginning of casting)
Immediately after the start of casting, the molten steel 5 is supplied to the empty mold 3 and the extraction of the molten steel 5 from the mold 3 is started, so that the molten metal surface fluctuation of the molten steel 5 in the mold 3 is large. When the molten steel surface level of the molten steel 5 in the mold 3 is large and the immersion nozzle 2 has a shallow immersion depth, the discharge holes 2a and 2b are located near the molten metal surface, so that the molten steel surface is greatly reduced. In this case, the discharge holes 2a and 2b may appear on the hot water surface and be exposed to the atmosphere. In this case, the molten steel flowing out from the discharge holes 2a and 2b comes into contact with the atmosphere, and the molten steel is oxidized. Although various components are contained in the molten steel, since aluminum is particularly easily oxidized, if the molten steel comes into contact with the atmosphere, aluminum oxide is precipitated in the molten steel, and the quality of the slab (product) is deteriorated. Therefore, immediately after the start of casting with large fluctuations in the molten metal surface, when the immersion nozzle 2 has a small immersion depth, the discharge holes 2a and 2b are likely to appear on the molten metal surface, and the above problem is likely to occur. Therefore, it is necessary to increase the immersion depth of the immersion nozzle 2 immediately after the start of casting.

  Further, if the immersion depth of the immersion nozzle 2 is set to the shallowest position at the time when the molten metal surface fluctuation is large, the discharge holes 2a and 2b of the immersion nozzle 2 are arranged at positions close to the molten metal 5 surface. Therefore, the molten metal surface tends to fluctuate more greatly due to the molten steel flow (upward flow Q shown in FIG. 1) flowing from the discharge holes 2a and 2b. Thereby, the molten powder 6 and the powder powder 7 on the molten metal surface are caught in the molten steel flow. The molten powder 6 and the powder powder 7 entangled in the molten steel cause surface defects such as scabs that occur in the product (a product obtained after rolling of the cast slab), resulting in a reduction in product quality.

  When the casting reaches a steady state, the amount of molten steel supplied to the mold 3 corresponds to the amount of molten steel withdrawn from the mold 3, so that the molten metal level of the molten steel 5 in the mold 3 hardly changes. . Therefore, when the casting reaches a steady state, the casting speed is stabilized (hereinafter, the casting speed when the casting speed is in the steady state is referred to as “constant speed”).

  Therefore, by increasing the immersion depth of the immersion nozzle from the start of casting until the casting reaches a steady state, it can be made less susceptible to fluctuations in the mold level during this period. Can be prevented from further fluctuations. Therefore, the immersion depth of the immersion nozzle is increased from the start of casting until the casting speed reaches a low speed (hereinafter referred to as the initial casting), and the immersion depth of the immersion nozzle during that time is, for example, about 300 mm. .

  When the molten steel surface fluctuation amount of the molten steel in the mold is not less than 0 mm / s and not more than 10 mm / s, the product (a product obtained by rolling a cast slab) has surface defects such as heges. It has been found that a product that hardly occurs and has no practical problem in quality can be obtained. However, when the fluctuation level of the molten steel in the mold exceeds 10 mm / s, surface defects such as scabs occur in the product (a product obtained by rolling a cast slab), and the product quality Has been found to cause practical problems. Therefore, by increasing the immersion depth of the immersion nozzle at the initial stage of casting, the amount of fluctuation of the molten metal surface in the mold is set to 0 mm / s or more and 10 mm / s or less.

(Immersion depth when continuous casting reaches a steady state after casting starts)
As described above, since the slag line of the immersion nozzle 2 (the portion in contact with the slag) is easy to melt, it is necessary to prevent the slag line from being in the same place in order to prevent breakage of the immersion nozzle 2. is there. Since the slag position is changed by changing the immersion depth, the slag line of the immersion nozzle 2 is changed by changing the immersion depth of the immersion nozzle 2.

In addition, when the immersion depth of the immersion nozzle is not changed during casting, the immersion nozzle usable time in which the immersion nozzle does not break is estimated from the following equation.
(Immersion nozzle useable time) = (Maximum immersion nozzle erosion possible amount) ÷ (melting speed)
Here, the above terms indicate the following.
・ Maximum amount of immersion nozzle erosion: Maximum amount of erosion in the thickness direction of the immersion nozzle that does not break the immersion nozzle. In addition, it is necessary to change the immersion depth of the immersion nozzle.

  When the continuous casting reaches a steady state, as described above, the molten steel surface level fluctuation in the mold hardly occurs. Further, since casting has just started, there is almost no deposit on the inner surface of the immersion nozzle 2, so that the inner diameter of the immersion nozzle 2 and the diameters of the discharge holes 2a, 2b have not substantially changed. Therefore, it is considered that the molten metal surface is hardly affected by the molten steel flow flowing from the immersion nozzle 2. Therefore, when continuous casting has reached a steady state after the start of casting, it is considered that even if the immersion depth of the immersion nozzle is reduced, the molten metal surface level hardly increases.

  Therefore, the immersion depth of the immersion nozzle when the continuous casting reaches a steady state is made shallower than the immersion depth of the immersion nozzle in the initial stage of casting. In addition, when continuous casting reaches a steady state, the molten metal level is considered to be the least, so the immersion depth of the immersion nozzle is set to the shallowest position of the immersion depth that is changed during continuous casting. Set.

(Immersion depth after continuous casting reaches steady state after casting starts)
As described above, in order to prevent the immersion nozzle 2 from breaking during continuous casting, it is necessary to change the slag line by changing the immersion depth of the immersion nozzle 2.

  Here, the immersion depth of the immersion nozzle when the continuous casting reaches a steady state is shallower than the immersion depth of the immersion nozzle in the initial stage of casting.

  By the way, as continuous casting proceeds, deposits such as aluminum oxide (reference numeral 10 in FIG. 1) adhere to the inner surface of the immersion nozzle. Since the amount of deposit increases as the continuous casting proceeds, the inner diameter of the immersion nozzle becomes smaller than that at the beginning of casting due to the deposit, or the substantial diameter of the discharge hole of the immersion nozzle is reached immediately before the end of casting. The molten steel flow injected into the mold is fast. Further, if molten steel is mainly injected from one of the two discharge holes, a large amount of molten steel is injected into the mold from the discharge hole, and the molten steel flow becomes faster. Therefore, since the molten steel flow in the mold is likely to fluctuate greatly due to the molten steel flow injected from the immersion nozzle just before the end of casting, the quality of the product is likely to deteriorate. Therefore, just before the end of casting, it is necessary to increase the immersion depth of the immersion nozzle as in the initial stage of casting.

  Therefore, after continuous casting reaches a steady state after the start of casting, the immersion nozzle is first set to a shallow immersion depth when continuous casting reaches a steady state, and then the immersion nozzle immersion depth is about to end casting. Is set deeply. Therefore, when the immersion depth of the immersion nozzle is changed after the point when continuous casting reaches a steady state after the start of casting, the immersion depth of the immersion nozzle is changed from a shallow state to a deep state.

  Here, when the immersion depth of the immersion nozzle is changed, the immersion depth is changed so that the slag lines do not overlap in order to prevent melting damage at the same location. In addition, as continuous casting progresses, factors affecting the fluctuation of the molten metal surface, such as the amount of deposits adhering to the inner surface of the immersion nozzle, will increase. Change in one direction from deep to deep.

  Note that the casting speed decreases just before the end of casting. Therefore, the amount of molten steel flowing into the mold within a unit time is small, the molten steel flow is slow, and the molten steel level in the mold is less likely to fluctuate. Sometimes. And if a hot water surface falls, immersion depth will become shallow. Therefore, the period during which the immersion depth is changed from a shallow state to a deep state is over a state in which continuous casting is in a steady state (the casting speed is constant) and close to the end of casting (hereinafter, sometimes referred to as a casting end stage). And

  Further, when the immersion depth is changed from a shallow state to a deep state, the immersion depth is gradually changed. Here, “gradually changing the immersion depth” means continuously increasing the immersion depth as shown in FIG.

  As a method for changing the immersion depth, in addition to the method for gradually changing, a method for changing in a stepwise manner can be mentioned. The immersion nozzle is melted most in the vicinity of the thickness center of the slag, and the amount of melt loss decreases as it approaches the upper end and the lower end of the slag (see FIG. 3A). Therefore, when the immersion depth is changed stepwise, even if the immersion depth is changed by the thickness of the slag by one change, the undissolved portion of the immersion nozzle (refractory) remains. Further, when the immersion depth is changed stepwise, the slag is located at the same place for a predetermined time, so that the erosion easily proceeds at the predetermined place. On the other hand, when the immersion depth is gradually changed, the vicinity of the thickness center of the slag is gradually moved with respect to the immersion nozzle, so that the undissolved portion does not remain and the damaged portion can be dispersed. The immersion nozzle (refractory material) can be used effectively (see FIG. 4B).

  When changing the immersion depth, for example, the immersion depth is changed by adjusting the amount of molten steel injected into the mold. In this case, the amount of molten steel injected into the mold can be adjusted by adjusting the opening / closing amount of a valve (not shown in FIG. 1) provided on the upper part of the immersion nozzle. In this embodiment, since the immersion depth is gradually changed, adjustment is performed by controlling the opening / closing amount of the valve.

  Moreover, since the immersion nozzle is attached to the tundish, the immersion depth can also be changed by changing the height position of the tundish. In this case, the immersion depth can be gradually changed by controlling the height position of the tundish.

  Note that, as described above, when the molten steel surface fluctuation amount of the molten steel in the mold exceeds 10 mm / s, there is a practical problem in the quality of the product, so the molten metal surface fluctuation amount is 0 mm / s or more and 10 mm / s. The immersion depth is changed so as to be s or less.

  In FIG. 2, as described above, the immersion depth (X in FIG. 2) between the start of casting and before the casting speed reaches a constant speed is deep. Further, the immersion depth when the casting speed reaches the constant speed of continuous casting (Y in FIG. 2) is the immersion depth (X in FIG. 2) from the start of casting until before the casting speed reaches the constant speed. ) Shallower. Thereafter, the immersion depth gradually increases from the time when the casting speed reaches the constant speed of continuous casting until the casting speed is constant and close to the end of casting. Moreover, the immersion depth (Y) when the casting speed reaches the constant speed of continuous casting after the start of casting is set to the shallowest position.

  In FIG. 2, the immersion depth (X) at the beginning of casting (after the start of casting) is deeper than the immersion depth (Z in FIG. 2) at the end of casting (immediately after the end of casting). The immersion depth may be shallower than the final immersion depth, or the initial immersion depth may be the same as the final immersion depth.

  Next, the immersion depth changing speed when the immersion depth is gradually increased from a shallow position will be described with reference to FIG.

(Immersion depth change speed)
3 (a) and 3 (b) show the time when the immersion nozzle is immersed in the molten steel in the mold. In FIG. 3, the mold and the molten steel are omitted, and only the immersion nozzle and the slag are shown. Further, the slag shown in FIG. 3 corresponds to the molten powder of this embodiment (reference numeral 6 in FIG. 2). FIG. 3A shows a case where the immersion depth of the immersion nozzle is changed stepwise, and FIG. 3B shows a process when the immersion depth of the immersion nozzle is gradually increased. . In addition, in FIG.3 (b), although the slag position is raising, the immersion depth of the immersion nozzle is changed in fact. In FIGS. 3A and 3B, the recess formed on the side surface of the immersion nozzle is a portion melted by slag.

Hereinafter, terms used to describe the immersion depth changing speed will be described.
-Melting width: It is the length in the dipping direction in which the dipping nozzle is melted by the slag, and corresponds to the thickness of the slag (d _{1 in} FIG. 3A).
-Melting speed: the amount of erosion per unit time in the thickness direction of the immersion nozzle-the maximum possible erosion amount of the immersion nozzle: the maximum amount of erosion in the thickness direction of the immersion nozzle that does not break during casting-the immersion depth Depth change speed: Movement speed and slag thickness when the immersion nozzle is moved in the immersion direction (direction to increase the immersion depth): Thickness of the slag floating on the molten steel (S in FIG. 3). The slag contains a complex oxide (SiO ₂ , CaO, F, etc.) that melts the immersion nozzle (refractory).

<Immersion depth change speed>
Increasing the immersion depth of the immersion nozzle raises the slag line relative to the immersion nozzle. As shown in FIG. 3B, while the slag rises by its thickness, the immersion nozzle is in contact with the slag the longest at the upper end of the slag before the rise (U in FIG. 3B). Therefore, the immersion nozzle is most easily melted at the upper end portion of the slag before rising (hereinafter sometimes referred to as “slag upper end portion”). Therefore, in order to prevent the immersion nozzle from being broken at the position (U), the amount of erosion at that position may be equal to or less than the maximum erosion possible amount of the immersion nozzle.

Therefore, the time during which the immersion nozzle can contact the slag at the upper end of the slag is obtained by the following equation.
(Time at which the immersion nozzle can contact the slag at the upper end of the slag)
= (Maximum possible erosion amount of immersion nozzle) ÷ (melting speed) (1)

  In order to allow the slag to rise by the thickness during the above time, from the above formula (1) and the thickness of the slag, (melting width (thickness of the slag)) / (immersion nozzle at the upper end of the slag The slag may be increased at a speed of the time during which contact with the slag is possible.

Since the slag rises by changing the immersion depth of the immersion nozzle, the minimum change speed of the immersion nozzle is expressed by the following equation.
(Immersion depth minimum change speed)
= (Scratch width (slag thickness)) ÷ {(Maximum possible erosion amount of immersion nozzle) ÷ (melting speed)} (2)

Here, under the following conditions:
Melting speed: 0.08 mm / min
Maximum immersion damage possible for immersion nozzle: 10mm
Melting width (slag thickness): 10 mm
From the above equation (2), the following immersion depth minimum change rate is obtained.
(Minimum immersion depth changing speed) = 10 ÷ {10 ÷ 0.08}
= 0.08
Therefore, it is necessary to change the immersion depth at 0.08 mm / min or more.

  Next, the maximum usable time of the immersion nozzle will be described with reference to FIG.

(Maximum usable time of immersion nozzle)
A refractory material (reference numeral 20 in FIG. 4) made of a material that is not easily melted by slag is attached to the outer portion of the lower portion of the immersion nozzle shown in FIG. Fig.4 (a) has shown the immersion nozzle before being immersed in the molten steel in a casting_mold | template. FIG. 4B shows a process when the immersion depth of the immersion nozzle is gradually increased. In FIG. 4B, the mold and the molten steel are omitted as in FIG. Moreover, the slag shown in FIG. 4B corresponds to the molten powder of this embodiment (reference numeral 6 in FIG. 2). Moreover, in FIG.4 (b), the recessed part formed in the side surface of an immersion nozzle is a part melt | dissolved by slag.

Hereinafter, terms used to describe the maximum castable time of the immersion nozzle will be described.
・ Maximum usable time of immersion nozzle: Maximum time that can be cast using one immersion nozzle ・ Fusion resistance range of immersion nozzle: refractory attached to the lower part of the immersion nozzle (reference numeral 20 in FIG. 4) ) In the immersion direction (D in FIG. 4)

In order to prevent breakage of the immersion nozzle, the slag is arranged in the range of resistance to damage of the immersion nozzle. When the slag is arranged at the lower end of the erosion resistance range (FIG. 4 (b) left) and the immersion depth is gradually increased, the slag becomes {(the erosion resistance range of the immersion nozzle) − (slag thickness)}. (The range (DS) in the left diagram of FIG. 4B) can be moved. Therefore, the maximum usable time of the immersion nozzle can be obtained from the following formula.
(Maximum usable time) = {(Damping resistance range of immersion nozzle) − (Slag thickness)} ÷ (Minimum change depth of immersion depth) (3)

When the following conditions are met:
Immersion nozzle resistance range: 180mm
Melting width (slag thickness): 10 mm
Immersion depth minimum change speed: 0.08 mm / min
From the above equation (3), the following maximum usable time of the immersion nozzle is obtained.
(Maximum usable time of immersion nozzle) = (180−10) ÷ {0.08}
= 2125
Therefore, the maximum usable time of the immersion nozzle is 2125 minutes.

  As described above, according to the method for changing the immersion depth of the immersion nozzle in the continuous casting according to the present embodiment, the immersion depth of the immersion nozzle 2 is changed during the continuous casting. It is possible to change the part where the immersion nozzle contacts the molten powder 6 on the molten steel. Therefore, it is possible to prevent the immersion nozzle from being melted at the same location, so that the life of the immersion nozzle can be extended. In addition, since the immersion depth is gradually changed, it is possible to disperse the melted portions and to effectively use the immersion nozzle (refractory) than when the immersion depth is changed stepwise. .

  In addition, the immersion depth of the immersion nozzle 2 in the molten steel 5 in the mold 3 is set to be deep at the beginning of casting when the molten steel surface fluctuation in the mold 3 is large and at the end of casting when the molten steel flow is fast. Therefore, even if the molten metal surface fluctuation is large, the discharge holes 2a and 2b formed in the lower part of the immersion nozzle 2 are not easily exposed to the atmosphere, so that oxidation of the molten steel injected from the immersion nozzle 2 can be prevented. Furthermore, since the immersion depth is set deep when the molten steel surface of the molten steel 5 in the mold 3 is large when the molten steel flow becomes fast, it is possible to prevent the molten metal surface variation from increasing. Therefore, a high quality product can be manufactured at the initial stage of casting when the molten metal surface fluctuation of the molten steel 5 in the mold 3 is large and at the end of casting when the molten steel flow is fast.

  Next, examples according to the present invention will be described.

(Examples 1-16, Comparative Examples 1-10)
Liquid level fluctuation at the beginning and end of casting when the immersion depth of the immersion nozzle is changed during continuous casting using a vertical bending type continuous casting machine for slabs and a curved bending type continuous casting machine for bloom And the quality of the cast slab was examined.

  In Table 1, as experimental conditions, it is added to the conditions of the continuous casting machine, the type and shape (width and thickness) of the slab cast by the continuous casting machine, the conditions of the immersion nozzle, and the molten steel in the mold. The fluorine (F) content of the flux is shown. The added flux exists as slag on the molten steel in the mold. Since the amount of erosion in the slag line of the immersion nozzle is affected by the fluorine content of the flux, Table 1 shows only the content of fluorine contained in the flux.

  Note that the inner diameter changes inside the immersion nozzle. Therefore, in Table 1, the range from the minimum diameter to the maximum diameter is described. Further, the outer diameter of the discharge hole of the immersion nozzle changes in a tapered shape from one opening to the other opening. Therefore, Table 1 shows the minimum to maximum opening of the discharge hole.

Table 2 shows conditions for changing the immersion depth of the immersion nozzle as experimental conditions. FIG. 5 shows an example of experimental conditions. FIG. 5 shows the relationship between immersion depth and time and the relationship between casting speed and time. Further, in FIG. 5, as the condition for changing the immersion depth, the immersion depth of the immersion nozzle is gradually increased from the time when the casting speed reaches a constant speed to the state where the casting speed is constant and closest to the end of casting. The condition when deepening is shown. Further, the X axis (time) in FIG. 3 is set to 0 when the slab in the mold starts to be drawn downward (when the slab in the mold starts to be drawn downward) (in FIG. Start drawing "). The experimental conditions in Table 2 will be described below.
(Casting speed)
Initial: a casting speed after the start of casting reaches a constant speed (casting reaches a steady state) to the casting speed of (Casting Initial) (r _b in FIG. _3).
Steady time: This is the casting speed (r _{S in} FIG. 2) when the casting speed reaches a constant speed (when the casting reaches a steady state).
(Time from casting start until casting speed reaches constant speed)
This is the time (T in FIG. 3) from the start of casting until the casting speed reaches a constant speed (casting reaches a steady state).
(Immersion depth change speed)
The change speed when the immersion depth of the immersion nozzle is gradually changed from the time when the casting speed reaches a constant speed after the start of casting to the state where the casting speed is constant and close to the end of casting (inclination g in FIG. 3). ). In Table 2, the immersion depth change amount (mm) per minute is shown. The minimum immersion depth changing speed is 0.08 mm / min from the above equation (2). Therefore, in this example and this comparative example, the immersion depth was changed at a speed of 0.08 mm / min or more.
(Maximum casting time)
In continuous casting, it is the time that one immersion nozzle can be used. The maximum castable time was determined from the above equation (3).
(Casting time)
It is the casting time in a present Example and a comparative example. The casting time was shorter than the maximum casting time.
In addition, when it casts without changing the immersion depth of an immersion nozzle, the maximum castable time when an immersion nozzle does not break is estimated from the following formula | equation.
(Immersion nozzle usable time) = (Maximum amount of immersion nozzle that can be melted) ÷ (Amount of damage)
Here, under the following conditions:
Melting speed: 0.08 mm / min
Maximum immersion damage possible for immersion nozzle: 10mm
From the above formula, the following usable nozzle nozzle time can be obtained.
(Immersion nozzle usable time) = 10 ÷ 0.08
= 125 (minutes)
Therefore, when casting is performed without changing the immersion depth of the immersion nozzle, it is estimated that the immersion nozzle breaks if the casting is continued for more than 125 minutes.
(Immersion depth)
・ Immersion depth from the start of casting until the casting speed reaches a constant speed:
The maximum immersion depth immersed in the molten steel in the mold of the immersion nozzle from the start of casting until before the casting speed reaches a constant speed (casting reaches a steady state) (hereinafter, this immersion depth is represented by X (FIG. 3). X).)
・ Immersion depth when casting speed reaches constant speed:
The maximum immersion depth immersed in the molten steel in the mold of the immersion nozzle when the casting speed reaches a constant speed after the start of casting (hereinafter, this immersion depth is referred to as Y (Y in FIG. 3)).
・ Immersion depth when casting speed is constant and closest to the end of casting:
The maximum immersion depth immersed in the molten steel in the mold of the immersion nozzle (hereinafter, this immersion depth is referred to as Z) when the casting speed is constant and closest to the end of casting (hereinafter sometimes referred to as the end of casting). (Z in FIG. 3). In this example and this comparative example, Z is the immersion depth after the casting time shown in Table 2 has elapsed.

Table 2 shows the experimental results when continuously cast under the above-mentioned experimental conditions. The molten steel surface fluctuations in the mold at the initial casting stage, the molten steel surface fluctuations in the casting mold at the end of casting, and the molten metal surface fluctuations. It shows the presence or absence of product quality degradation and the life of the immersion nozzle. In addition, the experimental result shown in Table 2 has shown the following.
・ Initial casting:
In the initial stage of casting, the molten steel surface in the mold hardly fluctuates (the amount of fluctuation of the molten metal is 10 mm / s or less) and the product has no practical problem in quality (hereinafter, the product is continuously cast. The product obtained by rolling the slab is shown as “○”.
When the molten steel surface fluctuation amount of the molten steel in the mold exceeds 10 mm / s in the early stage of casting, and a surface defect or the like occurs in the product manufactured at the end of casting, a practical problem occurs in the quality of the product. × ”.
・ End of casting:
At the end of casting, the molten steel surface in the mold hardly fluctuates (the amount of fluctuation of the molten metal is 10 mm / s or less) and has no practical problem in quality (hereinafter, the product is continuously cast) The product obtained by rolling the slab is shown as “○”.
At the end of casting, when the molten steel surface fluctuation amount of the molten steel in the mold exceeds 10 mm / s and surface defects occur in the product manufactured at the end of casting, a practical problem arises in the quality of the product. × ”.
・ Immersion nozzle life:
It has been found that if the immersion depth of the immersion nozzle is not changed during continuous casting, the immersion nozzle breaks after about 130 minutes from the start of casting. Therefore, in continuous casting, when the immersion nozzle did not break even after exceeding 125 minutes, it was determined that the life of the immersion nozzle was long, and “◯” was given.

  From Table 2, in Examples 1-16 and Comparative Examples 1-10, since the immersion depth of the immersion nozzle was changed during continuous casting, the slag line of the immersion nozzle was changed. Therefore, in Examples 1-16 and Comparative Examples 1-10, the lifetime of the immersion nozzle could be extended.

  Further, from Table 2, when the immersion depth (X) at the beginning of casting and the immersion depth (Z) at the end of casting are deeper than the immersion depth (Y) when the casting speed is constant (Example 1). In -16), the molten steel surface level of the molten steel in the mold hardly changed at the initial casting stage and the final casting stage. In addition, a product having no practical problem in quality could be manufactured in the initial casting stage and the final casting stage. However, in Comparative Examples 1 to 6, the initial immersion depth (X) was shallower than the immersion depth (Y) when the casting speed was constant. Therefore, since the molten steel was oxidized at the beginning of casting, or the powder level on the molten steel was entrained in the molten steel due to large fluctuations in the molten metal surface, the products manufactured in the early casting had a practical problem in quality. occured. Further, in Comparative Examples 7 to 10, since the immersion nozzle immersion depth was changed from a deep state to a shallow state from the time when the casting speed reached a constant speed to the end of casting, the immersion nozzle immersion depth at the end of casting was shallow. It was. Therefore, the molten steel surface in the mold fluctuated greatly at the end of casting, and the product manufactured at the end of casting had a practical problem in quality.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments and examples, and various modifications are possible as long as they are described in the claims. . For example, in this embodiment, the immersion speed when the immersion depth of the immersion nozzle is gradually increased from the time when the casting speed reaches a constant speed after the start of casting until the casting speed is constant and closest to the end of casting. The depth changing speed is constant, but the immersion depth changing speed may not be constant.

1 Tundish 2 Immersion nozzle 2a, 2b Discharge hole 3 Mold 6 Molten powder 7 Powder powder

Claims (1)

When molten steel is poured into a mold through an immersion nozzle and continuously cast,
From the start of casting until the casting speed reaches a constant speed, the immersion depth of the immersion nozzle into the molten steel in the mold is increased,
Thereafter, the immersion depth of the immersion nozzle when the casting speed reaches a constant speed after the start of casting is made shallower than the immersion depth of the immersion nozzle until the casting speed reaches a constant speed after the start of casting,
Immersion in continuous casting characterized by gradually increasing the immersion depth of the immersion nozzle from when the casting speed reaches a constant speed after the start of casting until the casting speed is constant and close to the end of casting. How to change the immersion depth of the nozzle.

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