JPH07300613A - Production of molten steel for continuous casting - Google Patents

Abstract

PURPOSE:To provide a producing method of molten steel for continuous casting by which the surface shape in a product plate is made excellent and the inclusion in the steel is drastically reduced for decreasing the internal defect can be achieved. CONSTITUTION:The molten steel is tapped after solidifying the slag by charging CaO into a converter, and the slag quantity on the ladle is decreased to <=0.1% of the tapped molten steel quantity. Successively, Al is uniformly spread on the slag in the ladle to reform FeO in the slag to <=3%. Thereafter, the deoxidation is executed by using an RH vacuum degassing apparatus or a simple type free board (CAS) and granular Mg alloy is added together with inert gas through a lance for gas blowing to control the composition of slag-based inclusion in the molten steel. By such a very simple method, since the internal defect can drastically be decreased, the effect on the industrial field is extremely large.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing molten steel for continuous casting in order 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. It is a thing.

[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.

Also, Kawauchi et al. Have applied for Japanese Patent Application No. 3-337922.
Nos, Japanese Patent Application No. Hei 4-14498, in Japanese Patent Application No. Hei 5-59333, for Al ₂ O ₃ inclusions, by forming a MgO by adding Mg alloy to molten steel, Al ₂ O
₃ · MgO, which reduces the interfacial energy between the oxide and the molten steel and reduces the clustering of the oxide, making Al ₂ O ₃ inclusions finer and harmless. Proposing a method.

XM + (1/2) yO ₂ → M _x O _y On the other hand, for slag type 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 an immersion hole.

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

[0006]

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, on the other hand, also reduces slag-based inclusions that cause internal defects. The purpose is.

[0007]

The present invention is configured to solve the above-mentioned problems, and when smelting molten steel for continuous casting, CaO is put into a converter to solidify the slag. After tapping into a ladle and reducing the amount of slag on the pot to 0.1% or less relative to the molten steel, and then spraying A1 evenly on the slag on the pot to modify FeO in the slag to 3% or less. , Deoxidizing, Ar, N ₂ by gas injection lance
The present invention is directed to a method for producing molten steel for continuous casting, which is characterized by controlling the composition of slag-based inclusions in molten steel by adding a granular Mg alloy together with an inert gas such as.

[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. .

In order to clarify the composition of inclusions which actually causes a problem, the present inventors conducted a compositional investigation of slag-based inclusions in cast slabs and inclusions of deep magnetic flaws in cold-rolled sheets. .
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, it is necessary to improve the composition of the inclusions floating in the molten steel to the high melting point side.

As a method for controlling the composition of inclusions in molten steel, we blow Mg source into molten steel to produce MgO, and combine this with slag-based inclusions in molten steel to control composition. Thought to do. Therefore, in a 20 kg controlled atmosphere melting furnace, CaO: 40-60%, Al ₂
O _3: 40 to 60% of oxide and molten steel oxide outside of this range is contained in a ratio of 50:50 (C: 0.0
1%, Si: 0.01%, Mn: 0.01%, P: 0.
001%, S: 0.004%, T.I. [0] = 10 pp
m) is melted, and a granular Mg alloy is added thereto by using a powder blowing device, and then MgO having an inclusion composition in molten steel is added.
The content was investigated. As a result, as shown in FIG. 3, it was found that the MgO content in the slag-based inclusions in the molten steel was uniformly 4% or more.

Here, the Japanese Patent Application No. 3-337922,
As proposed in Japanese Patent Application No. 4-14498 and Japanese Patent Application No. 5-59333, the S content in the molten steel before the addition of the Mg alloy is 0.005% or less in order to suppress the formation of MgS. Is desirable. Further, the timing of addition of the Mg alloy is preferably immediately after deoxidation with molten steel flow in order to combine with inclusions in molten steel. FIG. 4 compares the MgO content in inclusions in molten steel immediately after deoxidation and when a Mg alloy is added to the tundish.

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 melting of a molten steel for continuous casting, which contains 01 to 0.005% and Al: 0.04 to 0.06% and the balance Fe and unavoidable impurities. Three
In a 50t converter, CaO was charged into the furnace, adjusted to a predetermined carbon concentration, and tapped in a ladle. Al was sprayed on the ladle by a uniform spraying device to reduce FeO% in the slag to 3% or less. After that, deoxidation is performed in an RH vacuum degassing facility or a simple free board (CAS), and then an inert gas A is used by using an injection device.
A granular Mg alloy was sprayed together with r gas and cast with a curved continuous casting machine to form a slag cast piece having a thickness of 250 mm, and inclusions in the cast piece and the cold rolled sheet were investigated. Here, a comparison was made on whether or not the Mg alloy was blown. Detailed conditions and results are shown in Tables 1 and 2 (continued from Table 1).

[0016]

[Table 1]

[0017]

[Table 2]

Comparative Examples 1 and 2 are examples in which the Mg alloy was not added, and Comparative Examples 3 to 6 are comparisons with and without addition of the Mg alloy when solidification in the furnace was not sufficient. Further, Comparative Examples 7 to 10 show examples in which the presence / absence of the Mg alloy was compared when the reduction of the slag FeO% was not sufficient.
From the above examples and comparative examples, it is important to reduce the outflow amount of slag on the ladle from the converter, reduce the oxidizing power in the slag (reduce FeO%), and combine the technology of adding Mg alloy into molten steel. It can be seen that it is.

[0019]

As described in detail above, according to the present invention, it is possible to significantly reduce inclusions in steel during the production of molten steel for continuous casting, so that the surface texture of a product plate is improved. Becomes
There is an industrially useful effect of reducing internal defects.

[Brief description of drawings]

FIG. 1 is a diagram showing a composition of a slag-based inclusion in a cast slab.

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

FIG. 3 is a diagram showing the MgO content of slag inclusions in molten steel.

FIG. 4 is a diagram showing the relationship between the timing of adding Mg alloy into molten steel and the MgO content in inclusions.

Claims (1)

[Claims] 1. When smelting molten steel for continuous casting, slag is solidified by introducing CaO into a converter and tapped into a ladle, and the amount of slag on the pot is 0.1% of the molten steel.
After reducing to below, then A1 is sprinkled evenly on the slag on the pan to modify FeO in the slag to 3% or less, and then deoxidized,
Manufacture of molten steel for continuous casting characterized by controlling the composition of slag-based inclusions in molten steel by adding a granular Mg alloy together with an inert gas such as Ar and N ₂ by a gas injection lance Method.