JPH07300610A - Production of molten steel for continuous casting - Google Patents

Abstract

PURPOSE:To produce excellent molten steel for continuously cast slab scarcely containing harmful inclusion by tapping the molten steel in a converter into a ladle after solidifying a large part of molten slag in the converter and blowing flux having a specific composition into a small quantity of the molten slag in the ladle. CONSTITUTION:The molten steel is tapped into the ladle after raising the m.p. of the molten slag and solidifying the molten slag by adding CaO into the molten slag existing on the molten steel refined in the converter, and the quantity the molten slag flowing out with the molten steel into the ladle together is made small as <=0.1% of the tapped molten steel quantity. FeO in the molten slag is reduced to Fe by adding Al onto a small quantity of the molten slag to make the remained FeO <=3%, and thereafter, the flux having 0-40% CaO, 60-100% Al2O3, and 50-1000mum the grain diameter is blown together with inert gas such as Ar through a lance. Then, this flux is combined with CaO-Al2O3 base inclusion in the molten steel to make the inclusion harmless to the steel, and also major part of the harmless inclusion is floated up and separated from the molten steel. The molten steel for continuously cast slab having excellent quality as the slab for steel plate without defect caused by the harmful inclusion is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a molten steel for continuous casting, which is used to improve the surface shape of a product sheet and to greatly reduce the amount of inclusions in the molten steel 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, regarding slag inclusions, ladle slag winding is performed 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 reduces alumina-based inclusions to improve the surface quality of a product plate and also reduces slag-based inclusions that cause internal defects. The purpose is.

[0006]

The present invention is constructed to solve the above-mentioned problems, and its gist is to prepare a molten steel for continuous casting in a converter in which Ca is
After adding O to solidify the slag, it is tapped in a ladle, the amount of slag on the pot is reduced to 0.1% or less relative to the molten steel, and then Al is uniformly sprayed on the slag on the pot to slag. Medium FeO is reformed to 3% or less and Ar, N
CaO with inert gas ₂ such as: 0~40%, Al ₂ O
₃ : Grain size 50-1000 consisting of 60-100% composition
CaO-Al ₂ O ₃ slag inclusions floating in molten steel by spraying flux of μmφ, specifically Ca
O: 40 to 60%, Al ₂ O ₃ : 60 to 40% of a slag-based inclusion in a composition range, and is floated together to make the inclusion harmless and reduce the amount of the inclusion. It is in a method for producing molten steel for continuous casting.

[0007]

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 which actually causes 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 inclusions in the flaw detection of the magnetic particles is the same as that of the spherical shape in the slab stage, 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 ladle which has normally flown out during tapping from the converter is such that when it is wound into the 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, it is necessary to improve the composition of inclusions floating in the molten steel.

We have considered blowing powder into the molten steel as a method of controlling the composition of inclusions in 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, in a 20 kg controlled atmosphere melting furnace, CaO: 40-60%, Al ₂ O ₃ : 4
0-60% oxide and oxides outside this range are 50:
What is the amount of oxides of the composition in question and other compositions by melting the molten steel contained in the ratio of 50 and spraying the flux of the composition in the above range and the flux out of the range on the molten steel? investigated. As a result, as shown in FIG. 3, it was found that by spraying a flux of CaO <40% and Al ₂ O ₃ > 60%, the amount of inclusions of the composition in question decreased. The amount of inclusions will decrease even if a flux within the range of the problem is blown, but when the particles with small particle sizes coalesce and cannot float, the composition will still be the problem composition. It seems that spraying flux is more effective.

However, as shown in FIG. 4, it has been confirmed that the effect cannot be obtained when spraying with a particle size of less than 50 μm or more than 1000 μm. Presumably, if it is less than 50 μm, the probability of collision becomes small, and if it exceeds 1000 μm, it is considered that the harmlessness of the remaining residue without collision can not be achieved. On the other hand, with respect to the alumina-based inclusions that cause sliver defects, it is effective to reduce the oxidizing power in the slag. Therefore, after tapping, Al is uniformly sprayed on the slag on the pot to produce FeO in the slag. % Needs to be reduced to 3% or less.

Next, examples of the present invention will be described.

[0014]

[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 07 to 0.01%, Al: 0.04 to 0.06%, and the balance being Fe and inevitable impurities. 350t
In the converter, CaO is charged into the furnace, adjusted to a predetermined carbon concentration, tapped in a ladle, and Al is sprinkled on the ladle by a uniform spraying device to reduce FeO% in the slag to 3% or less. Then, using an RH vacuum degassing device or an injection device in a simple free board (CAS), CaO: 0 to 4 together with Ar gas which is an inert gas.
0%, Al ₂ O ₃ : A CaO-Al ₂ O ₃ based flux having a particle size of 50 to 1000 μmφ consisting of a composition of 60 to 100% is sprayed, then deoxidized, and cast by a curved continuous casting machine to 250 mm. A thick slab slab was used and the inclusions in the slab and the cold rolled sheet were investigated. Here, the comparison of the presence or absence of powder injection was investigated. Detailed conditions and results are shown in Tables 1 and 2 (continued from Table 1).

[0015]

[Table 1]

[0016]

[Table 2]

Comparative Example 1 shows an example in which solidification in the furnace was not sufficient, and Comparative Example 2 shows an example in which the slag FeO% was not sufficiently reduced. Further, Comparative Example 3 has CaO: 40-60
%, Al ₂ O ₃ : 40 to 60% of the product defect, and a flux having a composition which is a problem is sprayed. This is because there is a possibility that the same low melting point may collide with each other more easily, but although the amount of inclusions is reduced, the amount of inclusions of the composition in question cannot be reduced sufficiently.

Further, in Comparative Examples 4 and 5, the particle size of the powder to be blown was changed, but as shown in the examples, it is understood that the particle size of 50 to 1000 μm is appropriate.

[0019]

As described above in detail, according to the present invention, since molten steel for continuous casting having a significantly reduced amount of inclusions can be produced, a product sheet having excellent surface properties and reduced internal defects can be produced. The industrially useful effect of being able to do is exhibited.

[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 various fluxes are sprayed.

FIG. 4 is a diagram showing the influence of the particle size of the sprayed flux on the cleanliness of molten steel.

Claims (1)

[Claims] 1. When smelting molten steel for continuous casting, CaO is put into a converter to solidify the slag and then tapped into a ladle, and the amount of slag on the pot is 0.1% of the molten steel. Reduced to below, and then uniformly sprinkling Al on the slag on the pan to reform FeO in the slag to 3% or less, and CaO: 0-40 with an inert gas such as Ar and N ₂ by a gas injection lance. %,
Al ₂ O _3: particle size 50 consisting of 60% to 100% of the composition
A continuous process characterized by spraying a flux of 1000 μmφ to coalesce with CaO—Al ₂ O ₃ slag-based inclusions floating in molten steel to make the composition of the inclusions harmless and reduce the amount of the inclusions. Manufacturing method of molten steel for casting.