JPH11147164A - Method for continuously casting different kinds of steel in small cross sectional cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the nozzle clogging in a tundish nozzle and to secure the cleanliness of a cast slab at the time of sequential continuous casting of the different kind of steel in the small cross sectional cast slab of the kinds of steel having different Al contents with the same tundish. SOLUTION: At the time of executing the sequential continuous casting the different kinds of steel of the small cross sectional cast slab having <=0.1 m<2> cross sectional area of the cast slab 6 by using the same tundish 3, the tundish nozzle 7 is dipped into molten steel 1 in a mold 4 covered with mold powder 5 without blowing inert gas in the tundish nozzle 7, and at least one kind of steel is Al killed steel and the casting is executed in the order of the kind of steel having higher Al content. At this time, the difference of the Al contents in the kinds of steel of the previous casting and the following casting in the casting order is within 0.03 wt.% and then, these molten steels can stably be cast.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
It relates different steels continuous casting method using the same tundish ² following small section of the billet or bloom cast piece, more particularly, reduces the Al ₂ O ₃ produced by mixing of different grades in a tundish, Al ₂ O _{(3) The present} invention relates to a method for continuously casting different types of steel in which nozzle clogging of a tundish nozzle due to adhesion is prevented.

[0002]

^2. Description of the Related Art With the recent wide variety of small and large lots of steel materials, continuous casting of different types of steel using the same tundish is possible even in the continuous casting of billets and blooms having a small cross section of 0.1 m ² or less. The frequency of the continuous casting (hereinafter referred to as “continuous casting”) has become increasingly higher, and the continuous casting of different types of Al-killed steel and Si-killed steel has also been carried out.

[0003] In the continuous casting of Al-killed steel having a small-section slab, Al ₂ O generated by Al deoxidation is used.
₃ adheres to the inner wall of the tundish nozzle provided at the bottom of the tundish to inject molten steel into the mold, causing nozzle blockage. This nozzle blockage not only adversely affects the quality of the slab, but in severe cases necessitates stopping the pouring operation itself. Further, in the case of a small-section cast slab, when an inert gas is blown into the tundish nozzle to clean the adhering Al ₂ O ₃ , the molten gas surface in the mold fluctuates due to bubbles of the inert gas, and the mold is deformed. Powder is involved and the slab quality deteriorates. Therefore, since the casting is performed without blowing the inert gas into the tundish nozzle, nozzle clogging occurs more frequently.

[0004] Further, Si-killed steel has dissolved oxygen of 30 to 50 ppm, and reacts with a nozzle refractory, in particular, a C brick to cause nozzle melting. Therefore, in the continuous casting of different steel types of Al-killed steel and Si-killed steel in the same tundish, countermeasures against nozzle blockage and nozzle erosion have been taken.

For example, Japanese Patent Application Laid-Open No. Hei 7-204802 discloses that Ca is added to Al-killed steel in a range of 10 to 30 ppm in advance to convert inclusions into a CaO-Al ₂ O ₃ -based low melting point composite oxide. reforming and is liable site adheres to Al ₂ O ₃ tundish nozzle and the stopper tip MgO-
A method is disclosed in which a C brick or a ZrO ₂ -C brick is used, and the amount of oxygen dissolved in the Si-killed steel is in the range of 20 to 28 ppm. This makes it possible to continuously cast different types of steel for a long time without any nozzle blockage or nozzle erosion.

[0006]

However, according to JP-A-7-204802, Al-killed steel and Si
Even when successively casting different types of steel with a killed steel, the nozzle of the tundish nozzle may be blocked. As a result of investigating the situation where the nozzle clogging occurred, the nozzle clogging was performed by first casting a Si-killed steel having an Al content of less than 0.010 wt%, and then forming an Al-killed steel having an Al content of 0.010 wt% or more. When casting, it was found to occur frequently.

The present invention has been made on the basis of the above-mentioned findings, and an object of the present invention is to provide a small cross-section cast slab of a steel type having a different Al content while ensuring the cleanliness of the slab, and the nozzle blockage of the tundish nozzle. An object of the present invention is to provide a method for continuously casting different types of steel in the same tundish which can be cast stably without any problem.

[0008]

According to a first aspect of the present invention, there is provided a method for continuously casting a small-section slab of a different steel type, wherein the slab cross-sectional area is 0.1 m
^When casting different steel types of small-section slabs of ² or less using the same tundish one after another, without blowing inert gas into the tundish nozzle, the molten steel in the mold with the tundish nozzle covered with mold powder And at least one steel type is an Al-killed steel, and is cast in the order of the steel type having the highest Al content.

Further, the method for continuous casting of small-section cast slabs of different steel types according to the second invention is characterized in that, in the first invention, the difference between the Al contents of the steel types before and after the casting order is within 0.03 wt%. It is a feature.

Nozzle blockage in continuous casting of Al-killed steel is particularly likely to occur when the cross section of the slab is 0.1 m ² or less, because the inner diameter of the tundish nozzle becomes smaller. If the slab cross-sectional area exceeds 0.1 m ² , nozzle clogging can be prevented by blowing an inert gas into the tundish nozzle. Therefore, in the present invention, the slab cross-sectional area is limited to 0.1 m ² or less. In the present invention, Al
Killed steel is a steel type having an Al content of 0.010 wt% or more.

Further, in the present invention, the molten steel in the mold is covered with mold powder without blowing an inert gas into the tundish nozzle, and the tundish nozzle is immersed in the molten steel in the mold for casting. Since no inert gas is blown into the tundish nozzle, fluctuations in the molten steel surface in the mold due to the bubbles of the inert gas are eliminated, and the mold powder is not involved in the molten steel, and the cleanliness of the molten steel does not deteriorate. Also, since the tundish nozzle is immersed in the molten steel in the mold, air oxidation of the molten steel can be prevented, and fluctuations in the molten steel level in the molten steel can be suppressed, and the cleanliness of the molten steel can be ensured.

Then, successive castings of different steel types are carried out using the same tundish in the order of the steel type having the highest Al content. In the continuous casting of different steel types using the same tundish, in order to prevent continuous casting and to prevent the slag in the tundish from flowing into the mold, the molten steel of the pre-cast heat remains in the tundish, Inject molten steel into the tundish. Note that, in the temperature range of molten steel that equilibrates with Al in molten steel in Al-killed steel and Si in molten steel in Al-killed steel and stays in the tundish, the molten oxygen in Al-killed steel Dissolved oxygen is several ppm,
The dissolved oxygen in Si-killed steel is 30-50 ppm,
Dissolved oxygen is inversely proportional to the Al content or Si content.

[0013] When Al-killed steel is cast in the next casting using Si-killed steel containing a large amount of dissolved oxygen as a pre-cast, immediately after the injection of Al-killed steel, Al introduced into the tundish by the Al-killed steel is diluted to become Al. Although the reaction with dissolved oxygen does not proceed, due to the continuous injection of Al-killed steel,
When the amount of Al in the molten steel in the tundish reaches the critical concentration, the reaction with dissolved oxygen proceeds at a stretch, and a large amount of Al ₂ O ₃ is generated. This Al ₂ O ₃ cannot fully float in the tundish,
It flows into the tundish nozzle and causes nozzle blockage. At this time, the greater the Al content in the next casting, the greater the amount of Al brought into the tundish, and thus the greater the amount of Al ₂ O ₃ generated, which tends to cause nozzle clogging.

However, in the present invention, A having a high Al content is used.
Since the l-killed steel is pre-cast and the Si-killed steel is the next cast, the amount of dissolved oxygen brought into the tundish by the Si-killed steel with the injection of the Si-killed steel is diluted,
In addition, since Al of the Al-killed steel is present in a large amount in the tundish in comparison with the dissolved oxygen amount of the brought-in Si-killed steel, Al ₂ O ₃ is generated immediately after injection of the next-cast Si-killed steel, Al ₂ O ₃ generated is small,
Therefore, the nozzle is unlikely to be clogged by floating and separating in the tundish.

In order to prevent nozzle clogging, it is necessary to suppress Al ₂ O ₃ generated in the pre-casting and the next casting, that is, the difference in Al content of the steel type before and after the casting order within 0.03 wt%. More preferred.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1 is a schematic longitudinal sectional view of a continuous casting machine to which the present invention is applied, and FIG. 2 is a schematic longitudinal sectional view of a portion A shown in FIG.

In the figure, a tundish 3 constructed of an iron shell 11 and a refractory 12 inside the iron shell 11 is mounted on a tundish car (not shown) above a mold 4 and arranged. I have. At the bottom of the tundish 3, a tundish nozzle 7 is arranged at the opening of the steel shell 11 by fitting to the refractory 12, and a molten steel outflow hole 17 from the tundish 3 to the mold 4 is formed. . The cross-sectional area of the mold 4 is 0.1 m ² or less.

A stopper 9 is arranged above the tundish nozzle 7 to open and close the molten steel outflow hole 17. The stopper 9 is a stopper opening control device 1
By 0, the position is controlled, and the amount of molten steel flowing out to the mold 4 is appropriately controlled. The stopper opening control device 10
Receives the measurement signal of the eddy current level gauge 13 disposed immediately above the mold 4 and appropriately controls the amount of molten steel flowing out to the mold 4 according to the level of the molten steel 1 in the mold.

In the tundish nozzle 7 and the stopper 9 of this type, Al ₂ O ₃ easily adheres and accumulates at the joint between the tundish nozzle 7 and the stopper 9, which causes nozzle blockage. I know from experience. Therefore, it is preferable that the range including the joint on the side of the tundish nozzle 7 be the insert nozzle 8 fitted to the tundish nozzle 7, and the insert nozzle 8 and the stopper tip 9a be constructed of a boron nitride-containing material. Boron nitride-containing material has low wettability with molten steel, has the effect of preventing the deposition and deposition of Al ₂ O ₃ and suppressing the reaction with dissolved oxygen in Si-killed steel, preventing nozzle blockage and nozzle erosion. I do. Further, the tundish nozzle 7 and the stopper 9 other than the tip portion are made of a conventional Al ₂ O ₃ −
C refractory or high Al ₂ O ₃ refractory may be used.

The material containing boron nitride is, for example, BN: 22
~ 90wt%, AlN: 4-48wt%, SiC: 6 ~
30 wt%, BN: 22-90 wt%, AlN and / or
Or AlON: 4 to 48 wt%, SiC: 6 to 30 wt%
%, Al ₂ O ₃ : 4 to ₃₀ wt%, or BN: 20 to 7
0 parts by weight, AlN: 10 to 40 parts by weight, C: 10 to 30 parts
What is necessary is just to use the material etc. which mixed the weight part.

The upper opening of the tundish 3 is covered with a tundish cover 14, and an inert gas is supplied into the tundish 3 from an inert gas introduction pipe 15 penetrating the tundish cover 14. 3 can be made an inert atmosphere.

Prior to casting, the order of casting is determined from the types of steel to be successively cast with different steel types in the same tundish. The casting order is the order in which the Al content of the steel type is large. In the case where the Al content is the same, the casting order may be determined from those having similar contents other than Al in order to narrow the component mixing region.

The molten steel 1 is tapped from a steelmaking furnace (not shown) to a ladle 2, and a predetermined alloy element is added to adjust the components.
It is installed at a predetermined position above the tundish 3 and the molten steel 1 is injected into the tundish 3 to start the first casting of successively different types of steel.

The Al-killed steel is subjected to, for example, a treatment in an RH degassing facility (not shown) or a strong stirring treatment with a flux in a ladle refining facility (not shown) after Al deoxidation. Before casting, Al ₂ O ₃ is separated and removed from molten steel 1 as much as possible,
It is preferable that the molten steel 1 be cleaned. This is because, by reducing the amount of Al ₂ O ₃ of the Al-killed steel itself, nozzle blockage is more unlikely to occur. In addition, Ca of Al-killed steel
No treatment is performed, that is, it is better not to add Ca. C
This is because when a is contained, a problem arises in that the impact characteristics of the steel material caused by Ca deteriorate. For this reason, A
l After deoxidation, Al ₂
It is preferable to actively promote the floating and separation of O ₃ .

At the time of pouring into the tundish 3, the long nozzle 16 whose tip penetrates the tundish cover 14 is used to prevent the generation of Al ₂ O ₃ by air oxidation.
Before the injection into the tundish 3, an inert gas such as Ar gas is supplied from the inert gas introduction pipe 15 before the injection into the tundish 3 to start the injection with the inside of the tundish 3 as an inert gas atmosphere. When the molten steel 1 in the dish 3 rises, the tip of the long nozzle 16 is placed in the tundish 3
It is immersed in the molten steel 1 inside, and the air is shut off and injected. By doing so, the molten steel 1 can be cast without impairing its cleanliness.

When the molten steel 1 in the tundish 3 has reached a predetermined amount, the stopper 9 is raised by the stopper opening control device 10 and the molten steel 1 is introduced into the mold 4 through the molten steel outflow hole 17.
Inject. The molten steel 1 injected into the mold 4 is cooled and solidified to form a slab 6. When the molten metal surface of the molten steel 1 in the mold reaches a predetermined position above the discharge hole at the tip of the tundish nozzle 7, mold powder 5 is added to the mold 4 to cover the molten steel surface in the mold 4. Prevent air oxidation. Next, when the molten steel surface position becomes 50 to 200 mm from the upper end of the mold 4, the drawing of the slab 6 is started. Since the cleanliness of the slab 6 is deteriorated, no inert gas is blown into the tundish nozzle 7.

When the molten steel 1 in the ladle 2 is exhausted by continuing the casting, the ladle 2 is removed from a predetermined position above the tundish 3, and the ladle 2 containing another molten steel 1 is replaced above the tundish 3. It is installed at a predetermined position, and the casting into the tundish 3 is started by the above-described method to continue casting continuously. At that time, if the next casting heat is different from the previous casting heat,
It is preferable that the injection of the next casting heat into the tundish 3 is started after the molten steel amount of the pre-casting heat in the tundish 3 becomes 1/10 to 1/4 of the capacity of the tundish 3. When the injection of the next casting heat is started in a state where the molten steel 1 of the pre-casting heat remains in the tundish 3 in a large amount, as in the case of continuous casting of the same steel type, since the mixing area of the components is wide, the casting which is turned into waste is formed. This is because the pieces 6 are generated in a large amount, resulting in a decrease in yield.

By successively casting different types of steel in this way, it is possible to prevent the generation of Al ₂ O ₃ in the mixed zone due to the change of the type of steel, and as a result, without impairing the cleanliness of the molten steel 1. Casting can be stably continued in a state where the tundish nozzle 7 is not blocked.

It should be noted that the tundish nozzle 7 is not limited to the above type, but can be applied to a sliding nozzle type. In the case of the sliding nozzle type, the positions where Al ₂ O ₃ adheres and deposits are the upper nozzle immediately above the sliding nozzle and the part immersed in the molten steel 1 in the mold 4 of the immersion nozzle. Preferably, the site is constructed of a boron nitride-containing material.

[0030]

[Example 1] The slab size was 300 mm in diameter,
An embodiment of a curved billet continuous casting machine having the configuration shown in FIG. 1 and having six strands will be described.

The amount of molten steel in one heat is 250 tons, the capacity of tundish is 50 tons, and the speed of drawing slab is 0.9 m / m.
In, the casting time for one heat was about 80 minutes, and BN: 42 wt%, A
A material containing boron nitride of 1N: 45 wt% and SiC: 13 wt% was used, and Al ₂ O ₃ -C brick was used except for the tip of the tundish nozzle and the stopper.

The steel types to be cast are shown in Table 1.
As the three steel types of C to C, three heats of different steel types were continuously cast in the order of steel type B, steel type C, and steel type A in order of increasing Al content. In the continuous casting of different steel types, when the amount of molten steel in the tundish of the pre-casting heat became 7 tons, the injection of the next casting heat into the tundish was started. Steel type B and steel type C, which are Al-killed steels, were subjected to Al deoxidation during RH degassing after converter start-up, and the treatment was continued for 5 minutes after Al deoxidation. Note that C
a was not added. For comparison, only the casting order was changed, and successive castings of different steel types were performed in the order of steel type A, steel type B, and steel type C with the order of the C and Mn contents being low (comparative example). Table 1 also shows the casting order of Examples of the present invention and Comparative Examples.

[0033]

[Table 1]

In the embodiment of the present invention, no change in the opening degree of the stopper due to nozzle blockage and nozzle erosion during casting was observed over the entire range from the start of casting to the end of casting in three consecutive castings of different steel types. Also, after casting, the tundish nozzle was recovered and cut vertically, and the inner wall of the nozzle was observed.
No adhesion of Al ₂ O _{3 and} no nozzle erosion were observed.

On the other hand, in the comparative example, the opening of the stopper opened immediately after the start of the injection of the second heat into the tundish, and it was expected that Al ₂ O ₃ was attached to the inner wall of the nozzle. Further, as a result of longitudinally cutting the tundish nozzle after casting and examining the inner wall, it was confirmed that Al ₂ O ₃ was attached from the insert nozzle to the tip of the tundish nozzle.

[Example 2] In the same continuous casting machine as in Example 1, carbon steel having a C content of 0.10 to 1.0 wt% and an Al content of tr to 0.05 wt% was used. Continuously cast different steel types with the same tundish in order of increasing amount to investigate the stopper opening. The amount of increase in stopper opening after the component mixing zone (Δl) and the stopper opening during steady casting of pre-casting ( lo) and the percentage of Δl to lo was investigated as the nozzle blockage index. The result is shown in FIG.

As shown in FIG. 3, when the difference between the Al contents of the previous casting heat and the next casting heat is within 0.03 wt%,
No change in the stopper opening was observed, but it was found that when the difference in Al content exceeded 0.03 wt%, the stopper opening tended to increase. This is because the amount of generated Al ₂ O ₃ is proportional to the difference in the Al content between the previous casting heat and the next casting heat. Is set within 0.03 wt%.

[0038]

According to the present invention, be carried out different steels communicating s cast in the same tundish, it is possible to prevent the Al ₂ O ₃ produced in the mixing zone by switching steels, as a result, the molten steel Casting can be stably performed for a long time in a state where the nozzle of the tundish nozzle is not blocked while ensuring cleanliness.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of a continuous casting machine to which the present invention is applied.

FIG. 2 is a schematic longitudinal sectional view of a portion A shown in FIG.

FIG. 3 is a diagram showing a relationship between a difference in Al content and nozzle blockage in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Molten steel 2 Ladle 3 Tundish 4 Mold 5 Mold powder 6 Cast piece 7 Tundish nozzle 8 Insert nozzle 9 Stopper 10 Stopper opening control device 13 Eddy flow level gauge 14 Tundish cover 16 Long nozzle

Claims (2)

[Claims] 1. When performing continuous and continuous casting of different steel types of small-section slabs having a slab cross-sectional area of 0.1 m ² or less using the same tundish, without blowing an inert gas into the tundish nozzle. A tundish nozzle is immersed in molten steel in a mold covered with mold powder, and at least one steel type is Al-killed steel;
1. A method for continuously casting small-section cast slabs of different steel types, characterized by casting in order of steel type having the largest content. 2. The difference between the Al content of the steel type before and after the casting order,
2. The method for continuously casting a small-section cast slab of different steel types according to claim 1, wherein the content is within 0.03 wt%.