JPH0543929A - Pre-treating method for continuously casting sulfur-containing steel - Google Patents

Abstract

PURPOSE:To easily and surely prevent clogging of a nozzle by specifying CaO /(CaO+Al2O3)%: the control variable in double oxide of Al2O3 and CaO at the time of liquefying the Al2O3 by adding Ca into deoxidized and clean-treated molten steel. CONSTITUTION:At the time of continuously casting a sulfur-containing steel, by adding Ca into the deoxidized and clean-treated molten steel, the Al2O3 in the molten steel is liquefied as the double oxide with the CaO. In this pretreatment, (CaO/(CaO+Al2O3)%: the control variable) in the above double oxide, is adjusted to the value (range b) of >(CaO/(CaO+Al2O3)(%): equilibrium value) and <=[(CaO/(CaO+Al2O3)%: equilibrium value)-5%] in the equilibrium line (straight line 1 in the figure) of reaction in the formula decided with sulfur content in the molten steel and the molten steel temp. By this method, the clogging of nozzle caused by Al2O3 and CaS can easily and surely be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pretreatment method applied in continuous casting of sulfur-containing steel, and more particularly to the addition of Ca to deoxidized and cleaned sulfur-containing steel, The present invention relates to an improved technique of a method for preventing nozzle clogging by Al ₂ O ₃ by replacing Al ₂ O ₃ contained as an inclusion with a complex oxide with CaO.

[0002]

2. Description of the Related Art In continuous casting, especially in the case of small-section billet continuous casting, high melting point Al ₂ O ₃ mixed in the deoxidizing step is used.
Exists as an insoluble inclusion in the molten steel, it adheres to the vicinity of the tundish nozzle and gradually accumulates to cause nozzle clogging. Therefore, in order to avoid such a problem, Ca is added to molten steel to add Al ₂ O ₃ to a low melting point n.
Method of liquefying by changing to CaO · mAl ₂ O ₃ (Al ₂
O ₃ morphology control) is performed.

However, the sulfur content in steel is reduced to some extent (0.
When targeting high-grade steel strips that must be secured, the reaction of the following formula [I] proceeds to the right,

[0004]

[Equation 2]

CaS is produced. Since this CaS has a high melting point, it does not melt at the molten steel temperature, and it adheres and deposits on the nozzle portion, which rather causes nozzle clogging. Therefore, when adding Ca, it is necessary to perform control so as to make it difficult for the reaction of the above formula [I] to proceed to the right.

It has been confirmed that the equilibrium state of the reaction represented by the above formula [I] varies depending on the molten steel temperature and the sulfur content in the molten steel, but is represented by an equilibrium line as shown in FIG. 2, for example. That is, in FIG. 2, the region on the upper left of the straight line showing the equilibrium line of the reaction of the formula [I] shows the CaS stable region, and the region on the lower right shows the CaS unstable region. Therefore, in order not to cause the nozzle blockage by CaS, C
aS must be adjusted Ca addition amount so as unstable region, the Ca addition amount required decreases as becomes large sulfur content as is apparent from FIG. 2, except Al ₂ O
If the amount of ₃ is too large, that is, if the amount of Ca added is insufficient, it becomes impossible to prevent the nozzle clogging by Al ₂ O ₃ .

From such a point, by adding Ca, A
In controlling the morphology of l ₂ O ₃ , the C of the complex oxide to be produced depends on the molten steel temperature and the sulfur content of the molten steel.
The added amount of Ca is adjusted so that the aO / Al ₂ O ₃ ratio falls within the shaded area (CaS unstable area) in FIG.

[0008]

As can be seen from FIG. 2, the above-mentioned method is easy because the control range is wide for steel types having a low sulfur content (low [S] side in FIG. 2). Can be implemented. However, when the type of steel with a high sulfur content (high [S] side in FIG. 2) becomes narrow, the width of the CaS unstable region (hatched region) becomes narrower, so that all the inclusions in the molten steel fall within the hatched region. Things are very difficult.

The present invention has been made by paying attention to such a situation, and the purpose thereof is to confirm the fact as shown in FIG. 2 and to make a nozzle derived from Al ₂ O ₃ and CaS. It is intended to establish a pretreatment method capable of easily and surely preventing blockage.

[0010]

The constitution of the present invention which has been able to solve the above-mentioned problems is a pretreatment method carried out during continuous casting of sulfur-containing steel, which has been deoxidized and cleaned. Ca is added to the molten steel to convert Al ₂ O ₃ in the molten steel into C
When liquefied as a complex oxide with aO, [CaO / (CaO + Al ₂ O ₃ ) (%): control value] of the complex oxide is determined by the sulfur content in the molten steel and the molten steel temperature as shown in [I] below. Equation reaction equilibrium line

[0011]

[Equation 3]

[CaO / (CaO + Al ₂ O in
₃ ) (%): Equilibrium value] and {[CaO / (CaO + A
l ₂ O ₃ ) (%): equilibrium value] −5%} The gist of the invention is to adjust the value to the following.

[0013]

ACTION AND EXAMPLE The present inventors
_While conducting various experiments on the method of controlling the morphology of ₂ O ₃ , I learned the following facts. That is, Ca charged in molten steel is
It mainly reacts with oxygen of Al ₂ O ₃ (in addition to dissolved oxygen in molten steel and oxygen in slag) to form CaO, which is further combined with other Al ₂ O ₃ to form a low melting point composite oxide.
_{Since 2} O ₃ is originally mixed in molten steel in an amount of about 10 to 30 ppm, the liquid composite oxide produced is substantially spherical and dispersed in the molten steel. And the composite oxide is CaO
When it becomes excessive, CaO on the surface of the spherical composite oxide reacts with S in the molten steel, and CaS having a high melting point gradually precipitates as a solid on the surface of the spherical object. That is, CaS is nCaO.
Instead of being formed in a dispersed state in the mAl ₂ O ₃ composite oxide, it is formed in a skin-like state on the surface of the liquid composite oxide, gradually growing and becoming thicker.

Therefore, the present inventors have conducted a more detailed study on the relationship between such CaS formation and nozzle clogging. As a result, the proportion of the liquid phase (complex oxide) in the inclusions due to the small amount of CaS formation. However, when the CaS solid film formed on the liquid phase surface is thin, the nozzle blockage due to CaS does not occur even in the CaS stabilization region of FIG. It became clear that it could be carried out smoothly. However, since nozzle clogging occurs when the amount of CaS produced increases and the solid coating becomes too thick, it is necessary to clarify the critical criteria for preventing nozzle clogging due to CaS production. As a result of further research from this point of view, as shown in FIG. 1, [CaO / (CaO + Al ₂ O ₃ ) (%): control value] of the complex oxide was calculated as follows: ) In [C
aO / (CaO + Al ₂ O ₃ ) (%): equilibrium value] 5%
It has been found that the nozzle blockage due to CaS can be prevented by adjusting to a range not exceeding the above (region (b) of the first formula). That is, in Fig. 1, the region (a) is the CaS unstable region,
The region (c) is a CaS stable region, the region (d) is a nozzle clogging region due to Al ₂ O _{3, and the} region (b) is the control region defined by the present invention. Conventionally, the complex oxide composition has a region (c) ( It was thought that nozzle blockage due to CaS would occur at Ca loadings such as (b) region), but even in the CaS stable region (c), it was close to the equilibrium line (b) If the Ca charging amount is controlled so that the complex oxide composition is contained in the region, the nozzle clogging due to CaS can be prevented. As a result, the control range when Ca is charged is
It is possible to expand from (a) area to (b) area,
In particular, since the control range on the high [S] side can be widened, the control becomes extremely easy as compared with the conventional example.

Therefore, in carrying out the present invention, first, the equilibrium line is obtained in advance according to the molten steel temperature, and the complex oxide composition [CaO / (CaO + Al ₂ O) on the equilibrium line determined by the molten steel [S] content is obtained. ₃ ) (%): Equilibrium value]. Then, the generated composite oxide is
[CaO / (CaO + Al ₂ O ₃ ) (%): equilibrium value] and [[CaO / (CaO + Al ₂ O ₃ ) (%): equilibrium value]
The Ca charging amount may be adjusted so that it falls within the range of −5%} or less. By the way, FIG. 3 shows the results of examining the nozzle clogging state when several kinds of molten steels having different [S] contents were used and the complex oxide composition was variously changed depending on the Ca loading. As is clear from this figure, even when the complex oxide composition is in the CaS stable region, nozzle clogging is not seen as long as it falls within the region (b) defined by the present invention. However, when the complex oxide composition exceeds the region (b) and becomes CaO rich, nozzle clogging due to CaS occurs.

4 (A), (B), and (C) show CaO / Al ₂ O.
₃ shows the composition of inclusions when the Ca loading was adjusted so that the ₃ ratio became the symbol (a), (b) or (c) in FIG. As is clear from these figures, the composition of inclusions varies considerably depending on the sampling time, but Fig. 4 (A)
In (Comparative example with large nozzle clogging), a large amount of CaS is generated because the amount of CaO is too large, whereas in the case of FIG. 4B (example without nozzle clogging), FIG. The amount of CaS produced is considerably larger than in the case of the operation example in the CaS unstable region: conventional example), but the nozzle is not clogged.

As can be seen from the experimental examples shown in FIGS. 4 (A) to 4 (C), even when the same molten steel is used and the Ca charging amount is set to be the same, the composition of inclusions fluctuates in a considerably wide range. .. It is considered that this is because the distribution of inclusions in the molten steel is not necessarily uniform, the dispersion of Ca becomes insufficient depending on the stirring state at the time of charging Ca, and the distribution of CaO and CaS becomes nonuniform. Under such a situation, for example, even if the composition of the inclusions in a very narrow range on the high [S] side in FIG. 2 is determined, CaS is partially generated, and CaS is gradually deposited and deposited. It is thought to go. However, if the Ca charging amount is set so that the inclusion composition having an intermediate value in the control range including the region (b) according to the present invention is obtained, the nozzle clogging by CaS will occur even if the actual inclusion composition fluctuates considerably. Can be prevented.

[0018]

The present invention is constructed as described above, and C
By expanding the control range of the inclusion composition by charging a, it is possible to facilitate the pretreatment for preventing nozzle clogging, especially for high [S] steel, and to further improve nozzle clogging during continuous casting. It can be surely prevented.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a region of inclusion form control adopted in the present invention.

FIG. 2 is an explanatory diagram showing an example of a conventional inclusion form control.

FIG. 3 is a graph showing the relationship between the area of form control and the presence or absence of nozzle blockage.

FIG. 4 is a triangular graph showing the distribution of inclusion composition when the morphology control of FIG. 3 is performed.

Claims (1)

[Claims] 1. A pretreatment method carried out during continuous casting of sulfur-containing steel, wherein Ca is added to deoxidized and cleaned molten steel to convert Al ₂ O ₃ in the molten steel to CaO. When liquefied as a complex oxide of [CaO / (CaO + Al ₂ O ₃ ) (%):
[Control value] is determined by the sulfur content in the molten steel and the molten steel temperature. [CaO / (CaO + Al ₂ O ₃ ) (%): equilibrium value] and {[CaO / (CaO + Al ₂ O ₃ )
(%): Equilibrium value] −5%} A continuous casting pretreatment method for sulfur-containing steel, which is characterized by adjusting the value to the following value.