JPS5827020B2 - Method for cleaning molten steel in a tandate for continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cleaning molten steel in a tundish CC for continuous casting, and in particular, the inside of the tundish is divided into three chambers by two weirs. By devising the molten steel flow path,
The purpose is to strongly stir the molten steel in an intermediate chamber to promote agglomeration and enlargement of inclusions in the molten steel, particularly alumina clusters, and then to efficiently separate and remove the inclusions from the molten steel.

In the continuous casting method, the tundish is used as an intermediate container to guide molten steel from the ladle to the mold, but generally the molten steel poured into the tundish is free from interference caused by atmospheric oxidation and erosion of the tundish refractories. Easily contaminated with things.

If these inclusions are brought into the mold, it becomes difficult for them to float to the surface of the molten steel and impede the quality of the product.
It is important to take measures to prevent molten steel from being contaminated with inclusions as much as possible.

However, there are limits to contamination countermeasures, and a certain amount of inclusions are unavoidable, so various methods have been devised to actively remove inclusions within the tundish.

Specifically, the following method is adopted.

1) Increase the temperature of the molten steel injected into the tundish.

2) Increase the depth of molten steel in the tundish.

3) Use skimmers frequently.

4) Direction of flow of molten steel poured from the ladle upwards (rubbing).

5) Inject molten steel hair arc gas into the tundish.

6) Blow powder additives into the molten steel in the tundish.

However, each of these methods has the following problems.

In order to raise the injection temperature in step 1), it is necessary to raise the temperature of the molten steel during preparation for the converter, and as a result, the refractories in the converter, ladle, and tundish are melted and damaged, causing damage to each container. This will cause a decrease in lifespan.

2) Increasing the depth of molten steel in the tundish increases the equipment cost due to the extension of the equipment building height.

Excessive use of skimmers in 3) results in a one-step skimmer removal process and an increase in the cost of refractories.

In order to direct the flow of molten steel upward in 4), it is difficult to maintain the same upward flow for a long time due to the erosion and deformation of the refractory saucer in the steel receiving section, and this upward flow does not necessarily occur within the tundish. It's not something that gets recycled.

The blowing of argon gas in 5) causes a drop in the temperature of the molten steel,
In addition, this tends to cause slag and scum on the surface of the molten steel to be dragged into the molten steel.

In the case of blowing the powder additive (6), the tundish refractories are likely to be eroded and damaged by the fluorine component in the powder.

As described above, method 1 has many disadvantages, and therefore, there has been a desire to develop a simpler method that can efficiently remove inclusions instead of the method described above.

The present invention advantageously solves the above-mentioned problems by coagulating and enlarging fine alumina and inclusions with a low melting point in the molten steel received in the tundish, and then efficiently separating and removing the enlarged inclusions from the molten steel. The present invention provides a method for cleaning molten steel in a tundish for continuous casting.

That is, the present invention divides a tundish for continuous casting into three chambers, a steel receiving chamber, a stirring chamber, and an inclusion floating chamber, by two weirs arranged apart from each other, and while molten steel passes through these chambers in sequence, In order to clean the molten steel, the molten steel is introduced into the narrowly defined stirring chamber through the steel receiving chamber. The molten steel is strongly stirred through flows that collide with each other, thereby promoting agglomeration and enlargement of inclusions in the molten steel, and moving to the arched inclusion floating chamber. The above problem is solved by promoting the upward flow of molten steel by passing it through an upwardly sloping channel near the base of the weir, thereby floating the enlarged inclusions in the molten steel to the steel bath surface in the inclusion flotation chamber and separating them. It is meant as a means.

The present invention will be specifically explained below.

In the present invention, inclusions in molten steel injected into a continuous casting mold via a tundish are agglomerated and enlarged in the tundish, and then floated to the surface of the molten steel, thereby producing clean molten steel. The agglomeration and enlargement of the above-mentioned inclusions is achieved by strong stirring caused by mutual collision of the molten steel flows introduced into the stirring chamber.

That is, according to the present invention, the molten steel is narrowly defined from the steel receiving chamber through a plurality of molten steel channels that communicate the compartments provided near the bottom of the first weir that partition the steel receiving chamber and the stirring chamber. When introducing the molten steel into the stirring chamber, the exits of the flow channels in the stirring chamber are made to face each other so that the molten steel flows flowing into the stirring chamber collide with each other to generate turbulent flow, so that strong stirring of the molten steel is performed. It is to make it.

At this time, the surface of the molten steel is isolated from the atmosphere in the stirring chamber to prevent the surface of the molten steel from being oxidized in the same chamber (preferably by rubbing,
Inert gas such as argon gas can be blown into the same chamber to seal it, or flux can be used.

It should be noted that making the outlet on the stirring chamber side higher than the inlet on the steel receiving chamber side of the flow path, that is, providing the flow path with an upward slope, can promote the turbulent flow of molten steel in the stirring chamber. It is advantageous.

Next, the molten steel containing inclusions that has coagulated and enlarged due to vigorous stirring in the stirring chamber is guided into the inclusion floating chamber through a molten steel flow path provided near the base of a second weir that partitions the stirring chamber and the inclusion floating chamber.

At this time, the same chamber is divided into a steel receiving chamber and a stirring chamber.By increasing the capacity of the chamber, the flow of molten steel in the inclusion floating chamber becomes relatively smooth, and the agglomerated and enlarged inclusions are removed from the same chamber. It floats to the surface of the molten steel and is easily separated from the molten steel.

Furthermore, the molten steel flow path that connects the stirring chamber and the inclusion floating chamber is made higher at the floating chamber side outlet than the stirring chamber side inlet, that is, it is provided with an upward slope. This is extremely advantageous in terms of creating a flow and promoting the floating of inclusions.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 shows a plan view of an example of a continuous casting tundish suitable for carrying out the present invention, and FIG. 2 shows a line A in FIG. 1.
-A longitudinal section taken along A' is shown.

The molten steel injected from the ladle into the steel receiving chamber 2 of the tundish 1 at the point shown in FIGS. The mixture is guided into a narrow stirring chamber 4 with a small diameter.

In this case, the molten steel is guided through two branched channels in this example, and collides with each other at the outlet of the channels, creating a mixed state.

On the other hand, since a large amount of molten steel enters the narrow part of the stirring chamber from the flow path, the stirring of the molten steel in this part becomes strong and a turbulent state occurs.

The molten steel flow containing inclusions collides and mixes with each other at the outlet of the flow path, and is further subjected to significant stirring in the stirring chamber, so that the inclusions in the molten steel coagulate and enlarge in this region.

In this example, the inside of the stirring chamber was sealed with Ar gas to prevent the molten steel from coming into contact with the atmosphere.

The molten steel containing enlarged inclusions is then led to the subsequent inclusion floating chamber 6, where it is completely removed.

One or more channels 5 are used from the stirring chamber 4 to the inclusion floating chamber 6 and are angled slightly upward to help float the inclusions.

According to the present invention, dog-shaped inclusions in molten steel of 300 μm or more and medium-sized ones of about 50 to 250 μm are almost completely removed, and fine alumina particles of about 5 μm and low-melting point inclusions are removed in the form of clusters. It is characterized by the fact that it can be removed much more efficiently than conventional methods by increasing its size.

Although the tanditshu shown above shows the case where two molten steel channels are provided in the first weir, the present invention is not limited to this, and three or more channels are possible.

In addition, when sealing the stirring chamber with Ar gas Q as a barrier to the outside air (as I explained here, strong stirring occurs in the same chamber, so when flux is used, it is difficult to entrain it). Because there is.

In this case, flux may be used if the depth of the steel bath is sufficiently large and there is no concern about flux entrainment. The remaining inclusions are led to the flotation chamber where they are separated and removed.

Next, the results of investigating the amount of large inclusions, including alumina clusters, inside the slab of 40kg thick plate steel that was cast after cleaning the molten steel according to the present invention are compared with comparative examples and are indexed and shown in Table 1. show.

Inclusions were measured by X-ray transmission method.

The experimental conditions were: the distance between the two weirs (the inner diameter (Dl) and number (nl) of the flow channel of the weir on the closer side for the steel receiving part from the ladle, and the jetting angle of molten steel from the outlet of the flow channel (θ1). We tried to find the optimal conditions by changing the parameters.

The vertical angle θ2 between the flow path from the ladle near the steel receiving chamber and the bottom of the tundish is 0° (horizontal).
Tests were conducted by changing the angle between ~25°, but the influence on the inclusion index was small, so all experiments were conducted at 15°.

We also tested the number of flow paths from the stirring chamber to the inclusion floating chamber by varying the number n2 and the vertical angle θ3, and found good results. I went there.

The tanditshu used in the experiment had a length of 4.5 m.
The width was 600 TIrjn at the top, 400 rran at the bottom, and 650 mm deep.

No flux was used on the surface of the molten steel in the stirring chamber (an Ar gas seal was used to create a non-oxidizing atmosphere to prevent flux from being entrained during stirring).

The inclusion index has been improved as follows compared to the conventional method (tundish without weir).

Symbols 1) to 13) are cases in which molten steel for 40 kg steel is subjected to Ar bubbling treatment and then injected according to the method of the present invention, ■) is the conventional method, and 2) to 5) are molten steel from the flow path outlet. This is an example in which the ejection angle was changed between 0° and 90°.

The inclusion index decreases as θ1 increases and reaches 75°
gives the optimal value.

In this case, the inclusion index was reduced to about Σ compared to the conventional method.

Next, θ1 was fixed at the optimum value of 75°, and 6) to 8) were the cases in which the flow path inner diameter (Dl) was changed, and 9) to 10) were the cases in which the weir interval W was changed.

The inclusion index is smaller as Dl is smaller, and W is o,
It was found that the minimum value is shown near sm.

11) and 12) are comparative examples in which θ1 is fixed at 00, but because no collision of molten steel flows occurs, very good results are not obtained.

The optimum conditions were found from these results: θ1 was 75°, Dl was 50 mmφ, W was Q, 8 m, and Nl was two.

And symbol 13) is the optimal condition (experimental example),
The inclusion index was reduced to x compared to the conventional method.

Symbol 14) is the case where the Ar gas sealing step is omitted, but the inclusion index is still lower than that of the conventional method. A.

decreased to

From the above experimental examples, it has been demonstrated that making the molten steel flows collide with each other at the flow path exit from the steel receiving chamber to the stirring chamber to mix and stir more strongly is extremely effective in reducing inclusions.

This can also be understood from the stirring energy values in Table 1.

As mentioned above, a stirring chamber consisting of two weirs (one surrounded by two weirs) is provided inside the tandish, and by strongly mixing and stirring the molten steel passing through the chamber, the inclusions in the molten steel agglomerate and enlarge, and then the molten steel is separated from the molten steel. Efficiently removed.

The stronger the mixing and agitation, the greater the removal effect.

However, if the size is unnecessarily large, the molten steel is likely to be contaminated due to the erosion of the refractories in the tundish.
), or the number of flow paths (nl, n2), there are disadvantages such as insufficient hot water supply to the mold.

Therefore, in actual operation, it is desirable to decide the structure of the tundish based on the balance between such problems and the effect of reducing inclusions.

According to the present invention, without increasing the molten steel temperature higher than before and without changing the molten steel depth in the tundish,
Additionally, powder additives, Ar gas bubbling, etc. are removed (there is no need to do this).Inclusions in the molten steel passing through the tundish can be efficiently removed by a simple combination of weirs.

The present invention has the following advantages in carrying out continuous casting operations.

(1) Ar bubbling step can be omitted.

According to Example 14), even if Ar bubbling is omitted, the inclusion index is smaller than that of the conventional method of performing Ar bubbling and casting in a conventional tundish, and for this reason, the Ar bubbling step is omitted. can do.

As a result, it is possible to prevent the ladle slag from being mixed into the molten steel due to Ar bubbling, and it is also possible to prevent the return of Si from the slag and the decrease in A/l.

(2) The converter blowing completion temperature can be lowered by 30 to 50°C.

Temperature drop of steel inside ladle during Ar bubbling is 30 to 50
℃, but by omitting Ar bubbling, steel can be tapped at a low temperature only with this temperature valve.

As a result, the life of the furnace and the number of times the ladle is used can be improved.

(3) It is possible to shift to light RH treatment.

For steel types for which degassing is more important than deoxidation, hydrogen can be removed by light RH treatment, and deoxidation can be performed primarily by the method of the present invention, thereby shortening the preparation time.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a tundish for continuous casting suitable for carrying out the present invention, and FIG. 2 is a longitudinal sectional view taken along line AA' in FIG.

Claims (1)

[Claims] 1. Continuous casting tundishes arranged separately 2.
The molten steel is divided into three chambers: a receiving chamber, a stirring chamber, and an inclusion floating chamber, and in order to clean the molten steel as it passes through these chambers sequentially, it passes through the receiving chamber and narrows. The molten steel introduced into the stirring chamber thus defined is strongly stirred by colliding flows with each other due to the interaction of the plurality of channels provided near the bottom of the first weir that divides these compartments, thereby stirring the molten steel in the molten steel. The molten steel promotes agglomeration and enlargement of inclusions, and subsequently moves to the inclusion floating chamber.The molten steel is passed through an upwardly sloping flow path near the base of the second weir that partitions the stirring chamber and the inclusion floating chamber. A method for cleaning molten steel in a tundish for continuous casting, characterized in that enlarged inclusions in the molten steel are floated to the steel bath surface and separated in an inclusion flotation chamber.