JPS61183408A - Treatment of molten steel - Google Patents

Abstract

PURPOSE:To accelerate the uniform dispersion of an additive metal into a molten steel and to improve the yield thereof by blowing an alkaline earth metal or rare earth metal together with an inert gas into a molten steel which passes through the space part in a submerged gate provided in a molten steel flow passage. CONSTITUTION:The submerged gate 5 constituted by disposing refractory walls 3a, 3b having many molten steel passage holes P on the down stream side of a pouring pipe 2 is erected in contact with the base of the molten steel flow passage in a tundish 1 in the stage of casting the molten steel M accepted into said tundish 1 from a molten steel pan 7 through the pipe 2 via an overflow gate 8 into a casting mold 10 through a pouring port 9 provided with a stopper 11. The powder of an alkaline earth metal or rare earth metal element alone or the alloy thereof is blown together with the inert gas such as Ar through a tuyere 6 provided in the bottom surface position of the space part 4 into the molten steel M passing through the space part 4, by which the molten steel M is efficiently cleaned and the non-metallic inclusions therein are decreased.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to the use of an inert gas such as argon to inject alkaline earth metals or rare earth elements or their alloys into molten steel passing through a tundish or molten steel trough. This invention relates to improved techniques for blowing, cleaning steel, reducing non-metallic inclusions, and controlling their morphology.

[Conventional technology]

Impurities such as phosphorus and sulfur and nonmetallic inclusions contained in steel cause various defects, so reducing them is one of the important issues in the steel manufacturing field.

Under these circumstances, various measures have been taken to reduce impurities and nonmetallic inclusions in steel and purify the steel, as shown below.

(1) Reduction of impurities, nonmetallic inclusions, and gas components in molten steel by strengthening dephosphorization and desulfurization through hot metal pretreatment, strengthening refining in steelmaking furnaces and ladles, or vacuum degassing, etc.

(2) Complete sealing with inert gas to prevent air oxidation or nitridation of molten steel.

(3) Use of high-grade refractories with excellent resistance to molten steel to prevent impurity inclusions from being mixed in due to erosion of the refractories.

In addition, when continuous casting is performed after processing molten steel, it is normal for the molten steel to pass through a tundish and receive it in a tundish before injecting it into the mold, but impurities in the steel are agglomerated and separated immediately before casting. For this purpose, various efforts have been made to improve the structure of tanditshu, and proposals have been made, such as the installation of a one-stage weir or a two-stage weir. That is, by installing these weirs, it is possible to prevent A1ρ, Sin, etc. from being drawn in from the refractories at the bottom of the tundish, and to promote floating separation of nonmetallic inclusions by extending the residence time of molten steel and rectifying the flow.

In addition, a method of blowing inert gas from the bottom of the tundish to promote agglomeration and flotation separation of minute inclusions has been put into practical use in some cases. In addition, as one of the measures to improve steel quality, in order to promote finish deoxidation and desulfurization of molten steel, reduction of nonmetallic inclusions, and shape control, molten steel is made with alkaline earth metals such as Ca and rare earth metals such as La and Ce. Methods of adding elements are also known. These methods of adding modified metals include (a) ladle addition method, (b) tundish addition method, and (C) composite addition method to ladle and tundish. Modified metals are added as single metals or alloy powders. Alternatively, it is added in the form of soft iron coating or alloy wire, but in the case of ladle addition, most of the time it is an injection method using an inert gas as a carrier gas, and in the case of tundish addition, it is added directly into the tundish. The main methods are adding it to the injection tube and adding it to the injection tube.

The purpose of adding these reforming metals is to fulfill the functions of alkaline earth metals and rare earth metal elements, namely: 1) They have an extremely strong affinity with oxygen and sulfur and act as powerful deoxidizing and desulfurizing agents. ■Effects such as reducing nonmetallic inclusions such as MnS and Alρ1 that adversely affect the quality of steel materials, converting them to fine composite inclusions with a Cλ-AI-0 system composition, or reducing plastic deformation. The purpose is to demonstrate this, and a certain degree of effect has been obtained.

[Problem that the invention seeks to solve]

However, each of the above methods has unresolved problems, and it cannot be said that the expected effects are fully exhibited. For example, in the method of adding alkaline earth metals such as Ca as modifiers, these metals have high reactivity with oxygen and have very high vapor pressure, so there is a lot of loss during addition, making it difficult to sufficiently increase the yield. Can not. In addition, products such as oxides and sulfides do not form a solid solution in molten steel but are dispersed, so large grains are agglomerated and floated, and the products left in molten steel are finely divided to prevent segregation. It is important to disperse the particles with a uniform particle size, but if the amount of added Ci or the like is too large, the above-mentioned products may become huge inclusions and segregate, which may even worsen the steel quality.

In addition, when using the tundish addition method, a method is also adopted in which the added elements are uniformly dispersed and the yield is improved by adding a fixed amount as an alloy wire to the injection tube.
Since the additive elements for reforming described in +FI generally have high vapor pressure, a yield of only 10% can be obtained at most. Furthermore, even when using the ladle addition method or the inert gas stirring method, if the amount of inert gas injected is too large, the temperature of the molten steel will drop abnormally, and impurities in the inert gas will contaminate the molten steel. come.

Under these circumstances, the present invention makes it possible to add alkaline earth metal elements or rare earth metal elements to molten steel with a high yield and to disperse them uniformly, as well as to efficiently remove impurities in molten steel and clean it. The aim is to establish a technology that can improve the level of performance.

[Means for solving problems]

In the molten steel processing method according to the present invention, a submerged weir is provided in contact with the bottom surface of a molten steel channel, with a space formed between a pair of fireproof walls having a large number of molten steel passage holes.

The gist is that an alkaline earth metal or a rare earth metal element or an alloy is blown into the molten steel passing through the space of the submerged weir together with an inert gas.

[Effect]

In the present invention, continuous treatment of molten steel is carried out using equipment as shown in FIG. 1, for example. That is, in this example, Tanditsh 1
A submerged weir 5 [fireproof walls 3a and 3b having a large number of molten steel passage holes P are arranged so that a space 4 is formed between them] downstream of the charging position of the injection pipe 2. is provided in contact with the bottom surface of the tundish 1, and a tuyere 6 is provided at the lower surface of the space 4 for blowing the modified metal powder together with an inert gas. The molten steel M injected into the tundish 1 from the molten steel ladle 7 via the injection pipe 2 is
After passing through the fireproof walls 3a, 3b and the space 4 as shown by the solid arrows, it is successively cast into the mold 10 from the overflow weir 8 through the spout 9. In the figure, 11 indicates a stopper. 12 is a molten flux.

Here, in the present invention, granular alkaline earth metal or rare earth gold f11 (
(hereinafter sometimes referred to as added metals or modified metals) is blown into an inert gas stream to cause the added metals to efficiently react with impurities in the molten steel and to uniformly disperse them in the molten steel. That is, the added metal injected into the space 4 is uniformly dispersed in the molten steel MIllPj passing through the space 4 in a short time, and then passes through the molten steel passage hole P of the downstream refractory g3b together with the molten steel M. It flows downstream. The inert gas blown from the tuyere 6 exerts a stirring effect by bubbling in the space 4 and promotes the dispersion of the metal powder into the molten steel M, so that the molten steel M and the metal powder are separated. This increases the contact efficiency of steel and promotes the agglomeration of impurities and nonmetallic inclusions in the steel. The impurities thus formed pass through the passage holes P of the refractory walls 3a and 3b and float further toward the surface of the hot water. The impurities that float to the surface are absorbed by the molten flux covering the molten metal surface.

By employing such a blowing method, the following effects can be achieved.

(1) Since the amount of molten steel passing through the space 4 of the submerged weir 5 is almost constant, the amount added can be kept within a certain range by controlling the injection speed of the inert gas and the added metal. Moreover, uniform dispersion is possible.

(Since additive metals such as 21C1 have high vapor pressure, the nozzle immersion depth is 400 g in conventional molten steel ladle injection methods, etc.)
It is necessary to blow the metal while maintaining it at a temperature higher than g, and there is considerable evaporation loss of the added metal due to the dissipation of inert gas from the surface of the hot water.However, in the present invention, the method of blowing from the tuyeres at the bottom of the tundish. This eliminates the need for tedious work such as adjusting the immersion depth, and since the added metal efficiently contacts the molten steel in the space surrounded by the fireproof wall, evaporation loss is small and the added metal is Yield is improved.

(3) Since the contact between the inert gas and the molten steel is efficiently carried out within the space, the efficiency of coagulation and separation of impurities and non-metallic inclusions in the steel will also be improved. (4) In the conventional example, the added metal It is necessary to blow a large amount of inert gas in order to uniformly disperse the molten steel or to remove impurities from all the molten steel.As a result, the temperature of the molten steel decreases, the impurities in the inert gas are redissolved, and However, in the present invention, since the amount of inert gas blown can be significantly reduced, the above-mentioned problems can be solved at the same time.

FIG. 2 is a partially cutaway sketch showing an enlarged submerged weir 5 used in the present invention, showing a fireproof wall 3a in which a large number of molten steel passing holes P are drilled, and a fireproof wall 3a in which a large number of molten steel passing holes P are drilled. The fireproof walls 3b are arranged facing each other so that an appropriate space 4 is formed.

As is clear from the example in Figure 1, the molten steel to be treated is the fireproof wall 3.
The molten steel flows into the space 4 through the molten steel passage hole P in a, undergoes a predetermined treatment, and then flows out downstream from the molten steel passage hole P in the fireproof wall 3b. The added metal powder and inert gas bubbles blown into the fireproof wall 3
There is a tendency for the molten steel to flow out from the molten steel passage hole P in a short time.

Therefore, in order to increase the residence time of the added metal powder and inert gas in the space 4 and increase the treatment effect, it is advantageous to slightly increase the flow resistance of the fireproof wall 3b disposed on the downstream side. be. For this reason, in Fig. 2, two fireproof walls are overlapped as fireproof a3b, and as shown in the partial cross-sectional view of Fig. 3, the respective molten steel passing holes P and P are arranged in a staggered manner so that they intersect with each other. This increases overall distribution resistance. However, the means for increasing the flow resistance is not limited to this, and it is of course possible to take measures such as reducing the diameter of the molten steel passage hole P or reducing the number of holes formed.

The configuration of the submerged weir 5 characterized by the present invention, that is, the material and thickness of the fireproof walls 3a and 3b, the size and number of molten steel passing holes P, the volume of the space 4, etc. It should be selected and determined as appropriate depending on the processing amount, the amount of impurities in the molten steel, and the type and amount of added metal, and cannot be determined by law.

In the examples, an alumina-carbon type refractory is shown, but the material may be high alumina calcareous, magnesia, zirconia, or magnesia-carbon. Moreover, although the above description has been given of an example of processing within the tundish, it is of course possible to provide a similar submerged weir in the molten steel gutter to perform molten steel processing.

〔Example〕

The molten steel melted in a high-frequency atmospheric melting furnace and whose composition has been adjusted is
～C through a small tundish equipped with a submerged weir designed according to Fig. 3, and argon gas from the bottom of the space.
After the a-5i powder was blown into the ingot, it was cast into a mold, and the amount of nonmetallic inclusions in the ingot was examined.

The processing conditions were as follows.

<Processing conditions> Submerged weir construction fireproof wall material: Al, O,...92-93 Chi, Sin,
...1-2%, C...4-5 inches Fireproof wall dimensions: 140mm x 160IIjE x 3011JI
L molten steel passage hole: 1211jE'X 8 pieces x 6 stages fireproof wall 3
a... 1 piece, fireproof wall 3b... 2 layers stacked space thickness:
30u+ W4 Tsushio and submerged weir preheating temperature 2800℃ Molten steel processing amount = 150 Molten steel composition: Table 1 Added metals: (:, a-5i powder, 50-1509/
-FW) Inert gas: Argon, 5 to 101/'fi The amount of nonmetallic inclusions in the obtained ingot was measured according to JIS standards, and the results shown in Table 2 were obtained.

In addition, Figure 4 shows the results of investigating the amount of large nonmetallic inclusions contained in each ingot obtained above, and Figure 5 shows the distance from the surface of the same ingot and the amount of large nonmetallic inclusions. This shows the results of an experiment that investigated the relationship between the average number of objects, and if the invention is adopted, the amount of large nonmetallic inclusions can be significantly reduced. can be significantly reduced.

〔Effect of the invention〕

The present invention is constructed as described above, but the key point is that alkaline earth metals or rare earth metals are injected into a space surrounded by a fireproof wall that is provided in contact with the bottom of a molten steel flow path and has molten steel passage holes. Since these metals are blown into the molten steel and brought into contact with the molten steel within the space, the uniform dispersion of the added metals into the molten steel is promoted and the yield is improved, and the efficiency of agglomeration and separation of impurity inclusions by the inert gas is improved. As a result, molten steel with high purity can be obtained. Moreover, sufficient effects can be obtained with a small amount of inert gas injection, which not only provides economic benefits by reducing inert gas consumption, but also reduces the problem of abnormal drop in molten steel temperature and re-contamination due to impurities in the inert gas. It is possible to enjoy various practical benefits, such as a significant reduction in the cost.

[Brief explanation of drawings]

FIG. 1 is a schematic vertical sectional view showing an embodiment of the present invention. Fig. 2 is a partially cutaway diagram illustrating the submerged weir used in the present invention, Fig. 3 is a partially sectional view illustrating the fireproof wall that constitutes the submerged weir, and Figs. 4 and 5 are diagrams obtained through experiments. It is a graph showing the number of large inclusions in an ingot. 1... Tanditshu 2... Injection pipes 3a, 3b.
... Fireproof wall 4 ... Space 5 ... Submerged weir
6... Tuyere Figure 4 BC Treatment method Distance from the ingot surface layer (ff)

Claims (1)

[Claims] A submerged weir formed by forming a space between a pair of fireproof walls having a large number of molten steel passing holes is erected in contact with the bottom of the molten steel flow path, and the molten steel passing through the space of the submerged weir is A method for treating molten steel, which comprises blowing an alkaline earth metal or a rare earth metal element or an alloy together with an inert gas.