JPH07300611A - Production of molten steel for continuous casting - Google Patents

Abstract

PURPOSE:To produce excellent molten steel for continuously cast slab by tapping the molten steel into a ladle after solidifying molten slag in a converter, discharging it into a tundish after deoxidizing a small quantity of the molten slag in the ladle and blowing flux having a specific composition into the molten steel thereby removing remaining inclusion in the molten steel. CONSTITUTION:The molten steel is oxygen-refined in the converter and tapped into the ladle after raising the m.p. of the molten slag in the furnace and solidifying the molten slag by blowing MgO, and major part of the solidified slag is allowed to remain in the furnace. Al is added into a small quantity of the molten slag being <=0.1% of the tapped molten steel quantity in the ladle to reduce FeO to <=3%. Successively, the molten steel is discharged into the tundish after deoxidizing the molten steel in an RH type vacuum degassing apparatus, and the flux having 0.66-1.5 ratio of CaO/Al2O3 and the composition composed of <=70% the total of CaO and Al2O3 and the balance SiO2 or MnO and 1450-1550 deg.C the m.p. and 50-200mum the grain diameter is blown into the molten steel. The continuous casting operation is executed after combining the inclusion with the flux in the molten steel and floating up and separating. The continuously cast slab for steel plate having excellent surface characteristic without internal defect caused by inclusion is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten steel for continuous casting for improving the surface shape of a product plate and reducing inclusions in the molten steel in a continuous casting process in order to reduce internal defects. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Inclusions that are generated or generated in a steelmaking process and are not removed but cause product defects include:
There are roughly three types: alumina-based, slag-based, and powder-based. Of these, the alumina-based inclusions are (1)
It is generated in the deoxidation by the reaction shown in the formula and the subsequent reoxidation reaction.For the conventional deoxidation product, in order to prolong the flocculation separation time by aggregation of oxides and coalescence in the deoxidation step, Steel deoxidation method or RH in which a deoxidizer is added
A method has been adopted in which strong agitation in the treatment is performed to promote floating separation of inclusions. In addition, JP-A-1-18046
As seen in Japanese Patent Publication No. 6, the deoxidizers Al and Ca are used during deoxidation.
A method has also been proposed in which a deoxidized product is promoted to be floated as a CaO—Al ₂ O ₃ -based low melting point material by adding a binder and a fusion product of O and CaF ₂ . Furthermore, regarding the reoxidation reaction product, it is said that it is effective to reduce the degree of reoxidation of the slag on the pan, and the (T.Fe)
So-called slag reforming has been performed.

XM + (1/2) yO ₂ → M _x O _y On the other hand, for slag inclusions, ladle slag winding by introducing a slag stopper or a floating valve as a cleaning measure at the pot seam. Attempts have been made to prevent the tundish slag from being struck by preventing it from being entangled or by making a dip opening.

With respect to powder inclusions, measures such as increasing the viscosity of the powder and low speed casting operation have been taken.

[0005]

However, at present, most of the product defects in the material for food cans are flange cracks caused by Al-Ca-O (slag-based) inclusions. The current situation is that measures have not been taken sufficiently. The present invention provides a method for producing molten steel for continuous casting, which can reduce alumina-based inclusions in a product plate to improve surface properties and also reduce slag-based inclusions that cause internal defects. That is the purpose.

[0006]

The present invention is constructed to solve the above-mentioned problems, and its gist is to melt molten steel for continuous casting in a converter in the final stage of blowing with MgO. After the composition of the converter slag is controlled to the high melting point side by blowing to solidify the slag, it is tapped in a ladle, and the amount of slag on the ladle is reduced to 0.1% or less relative to the molten steel, In addition, the MgO content in the converter slag is uniformly increased to 4% or more, and then Al is evenly dispersed on the slag on the pan to modify FeO in the slag to 3% or less, and then deoxidation is performed to obtain The molten steel thus obtained is further fed into a tundish, and the molten steel has a CaO: Al ₂ O ₃ ratio of 0.66 to 1.5,
The content of both is 70% or less, the content of SiO ₂ and MnO is 30% or more alone or in combination, and the melting point is 1450.
It is a method for producing molten steel for continuous casting, characterized in that a flux having a particle diameter of 50 to 200 μmφ within ˜1550 ° C. is sprayed, the flux is coalesced and floated with inclusions in the molten steel, and the amount of inclusions is reduced.

Furthermore, in the tundish, Ca
The ratio of O: Al ₂ O ₃ is 0.66 to 1.5, the content of both is 70% or less, the content of SiO ₂ and MnO is 30% or more alone or in combination, and the melting point is 1450 to 1550.
The purpose of the method for producing molten steel for continuous casting is to reduce the amount of inclusions by spraying a flux of 50 to 100 μmφ within a temperature of 0 ° C. to float the inclusions in the molten steel.

[0008]

The defects caused by inclusions are roughly classified into the following two types. One is sliver flaws caused by alumina-based inclusions, and the other is flange cracks and pinhole defects caused by slag-based inclusions such as CaO—Al ₂ O ₃ . As mentioned above, the origin of the slag-based inclusions is considered to be the inclusion of converter slag during tapping, shaving of the slag on the pan due to molten steel flow, and the inclusion of the slag on the pan at the pan joint. .

The present inventors conducted a compositional investigation of slag-based inclusions in cast slabs and inclusions of magnetic particle flaw detection defects in cold-rolled sheets in order to clarify the composition of inclusions that actually cause problems.
Inclusions in the slab and inclusions in the magnetic particle flaw detection portion are CaO-
It is an Al ₂ O ₃ binary system, and the compositions obtained by the investigation of both are plotted on the CaO—Al ₂ O ₃ binary system phase diagram in FIGS. 1 and 2. FIG. 1 shows the composition of the slab inclusions, and FIG. 2 shows the composition of the magnetic particle flaw detection defect inclusions. The slab inclusions are further shown as spherical and amorphous shapes. The existence ratio of the spherical and amorphous shapes of the inclusions in the slab is 50:50.

As can be seen from these figures, the composition of the inclusions in the flaw detection of the magnetic particles is the same as that of the spherical shape at the stage of the cast piece, and the composition is CaO: 40-60%, Al.
₂ O ₃ : Within the range of 40 to 60%. That is, at the casting stage, inclusions (CaO <40%, Al ₂
It was found that, despite the presence of O ₃ > 60%), these are not actually defects.

Here, the composition of the slag on the pot that normally flows out during tapping from the converter is such that when it is rolled into molten steel, the components such as FeO, SiO ₂ , MnO in the slag are reduced by Al in the molten steel. Be sure, CaO: 40-60%, Al
₂ O ₃ : It has a composition within the range of 40 to 60%. Therefore, as proposed in Japanese Patent Application No. 6-004235, it is necessary to improve the composition of the slag on the pot in the converter to the high melting point side.

Further, as a method for controlling the composition of inclusions in molten steel, we considered blowing powder into the molten steel.
That is, it was considered to blow the powder out of the composition range, which is a problem, to float it together with the problematic low melting point slag-based inclusions floating in the molten steel. Therefore, CaO: Al ₂ O ₃ = 0.66 in a 20 kg controlled atmosphere melting furnace.
˜1.5 oxide and CaO: Al ₂ O ₃ <0.6
6, powder that melts molten steel containing oxides of> 1.5 at a ratio of 50:50 and has a problematic composition (C
aO-: the Al ₂ O ₃ = 0.66~1.5) is in the molten steel because liquid inclusions low-melting, considered a low melting point flux which the collision tends liquid. However, when floating without collision, other components are reduced by Al so that the composition of the problem does not occur, and the ratio of CaO: Al ₂ O ₃ is 0.66 to
The ratio of CaO: Al ₂ O ₃ is 0.
66 to 1.5, but the content of both is 70% or less, and the rest is Al ₂ O _{3 when} reduced such as SiO ₂ and MnO.
A flux containing 30% or more of the above composition alone or in combination (hereinafter referred to as flux A) was sprayed, and the amount of the oxide of the composition in question and the composition other than that was investigated. As a result, as shown in FIG. 3, it was found that the amount of inclusions in the composition in question (CaO: Al ₂ O ₃ ratio of 0.66 to 1.5) was reduced. This flux is easy to combine as long as it is liquid, and even if it remains without being completely floated, other components of SiO ₂ , MnO are finally reduced by Al, and the composition of the problem deviates. Target.

The addition timing is effective in the ladle and in the tundish, but it is also expected to be effective in the non-steady part (pot joint), so the addition in the tundish is preferable. However, as for the particle size of the flux to be sprayed, if the flux cannot be completely floated and remains in the molten steel, Japanese Patent Application No. 6-
As proposed in No. 13062, it has been found that it is necessary to keep the size to 200 μm or less, which is not a problem, and when it is less than 50 μm, the effect cannot be obtained. This is probably because the probability of collision decreases.

In order to reduce the absolute amount of alumina-based inclusions that cause surface flaws (sliver flaws), it is effective to reduce the oxidizing power in the slag. Al is evenly dispersed on the slag and FeO% in the slag
Should be reduced to 3% or less. Next, examples of the present invention will be described.

[0015]

[Example] As a target component, C: 0.03 to 0.05
%, Si: 0.08 to 0.15%, Mn: 0.20
0.50%, P: 0.007 to 0.01%, S: 0.0
The present invention was applied to the production of molten steel for continuous casting containing 01 to 0.005%, Al: 0.04 to 0.06%, and the balance Fe and unavoidable impurities. 350
In the converter of t, the composition of the converter slag is controlled to the high melting point side by solidifying the slag by performing MgO blowing in the final stage of the blowing, and then the slag is tapped and the amount of slag on the ladle is adjusted. 0.1%
The FeO% in the slag was reduced to 3% or less by reducing the content to below, and uniformly increasing the MgO content in the converter slag to 4% or more. . Next, deoxidation was performed in an RH vacuum degassing device or a simple free board (CAS), and then the mixture was supplied into a tundish so that the ratio of CaO: Al ₂ O ₃ was 0.
66 to 1.5, the content of both is 70% or less, and the balance is SiO ₂ or MnO alone or in combination of 30
% Or more, the melting point is within the range of 1450 to 1550 ° C., and the flux having a particle size of 50 to 200 μm is sprayed and cast by a curved continuous casting machine to form a slab cast with a thickness of 250 mm. I conducted a physical survey. Here, a comparative study was conducted on the presence or absence of MgO blowing at the end of converter blowing and the presence or absence of flux blowing. Detailed conditions and results are shown in Table 1, Table 2 (continued from Table 1), Table 3 and Table 4 (continued from Table 3).

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

[0019]

[Table 4]

Comparative Examples 1 to 5 show the relationship between the solidification in the furnace and the method of introducing the composition control material and the presence or absence of flux addition. Further, Comparative Examples 6 to 8 show the results when MgO blowing was performed in the final stage of the converter, and no flux was added or a flux particle size outside the range was blown.

[0021]

As described above in detail, according to the present invention, since it is possible to obtain the molten steel for continuous casting in which the amount of inclusions is greatly reduced, it is possible to obtain a product sheet having excellent surface properties and significantly reducing internal defects. And an industrially useful effect of being able to provide.

[Brief description of drawings]

FIG. 1 is a graph showing the composition of slag inclusions in a cast slab.

FIG. 2 is a graph showing the composition of inclusions in a magnetic particle flaw detection portion.

FIG. 3 is a diagram showing cleanliness of molten steel when flux is added.

FIG. 4 is a diagram showing the relationship between the flux particle size and the cleanliness of molten steel.

Claims (1)

[Claims] 1. When melting molten steel for continuous casting,
It is converted by performing MgO blowing at the end of blowing in the furnace.
The slag was solidified by controlling the composition of the furnace slag to the high melting point side.
Steel is tapped in the ladle, and the amount of slag on the ladle is 0.1 with respect to the molten steel.
% Or less, and reduce the MgO content in the converter slag
Raise to 4% or more evenly, then evenly on the slag on the pan
Al was sprinkled to modify FeO in slag to 3% or less.
After that, deoxidation is performed and the resulting molten steel is further placed in the tundish.
Is supplied to the molten steel and CaO: Al is added to the molten steel. ₂O₃Ratio of 0.66
.About.1.5, the content of both is 70% or less,
₂, MnO alone or in combination of 30% or more,
The particle size is 50-200μ with the point within 1450-1550 ° C.
Spray a flux of mφ and insert the flux into the molten steel.
Characterized by floating together with existing objects and reducing the amount of inclusions
And a method for producing molten steel for continuous casting.