JPH07207327A - Production of molten steel for continuous casting - Google Patents

Abstract

PURPOSE:To reduce inclusion in a continuous casting process. CONSTITUTION:In a converter, MgO content in the converter slag is uniformly raised to >=4% by executing MgO blowing at the end stage of the blowing to control the composition of the flow-out of the slag from the converter to the high m.p. side. Further, the slag is hardened by the effect making the slag the high m.p. to prevent the flow-out of the slag during steel tapping, and the slag quantity on a ladle is reduced to <=0.1% to the molten steel quantity. Successively, Al is uniformly scattered on the ladle, and after modifying FeO% in the slag to <=3%, Ar is blown from the ladle bottom part. Deoxidation is executed in the inert gas atmosphere while avoiding the contact with the slag to reduce the inclusion in the molten steel. By this method, since the quality of the molten steel is remarkably improved, the great effect on the industry is obtd.

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 for reducing the amount of inclusions, improving the surface shape of a product plate, and reducing internal defects in the continuous casting process. It is about.

[0002]

2. Description of the Related Art In the steelmaking process, there are roughly two types of inclusions as follows. One is an alumina-based inclusion and the other is a slag-based inclusion. Alumina-based inclusions are minute inclusions generated by the reaction shown in the following formula (1) by adding a deoxidizer to remove oxygen in molten steel after oxidizing and refining the hot metal in a converter.

XM + (1/2) O ₂ → M _x O _y (1) Further, other than when the deoxidizing agent is charged, minute oxides of these oxides are generated by the subsequent reoxidation reaction and are removed. Instead, it remains in the slab. As a result, this is one of the causes of surface defects in products manufactured from thin plates after cold rolling (steel plates for automobiles, food cans). In addition, slag-based inclusions involve suspended slag that inevitably flows out during converter steel tapping, and the slag on the pan is then drawn into the molten steel due to molten steel flow. Has become.

Conventionally, in order to reduce the amount of these inclusions, Japanese Patent Application Laid-Open No. 1-18 has proposed a fine alumina-based inclusion.
As can be seen in Japanese Patent No. 0466, a deoxidation product produced by adding a combination of Al and CaO and CaF ₂ as a deoxidizing agent and a fusion product at the time of deoxidation is converted to CaO-Al.
A method of promoting levitation as a low melting point slag inclusion of ₂ O ₃ has been proposed.

Further, in the deoxidizing step, in order to give a long floating separation time due to the aggregation and aggregation of oxides, a de-oxidizing method such as de-oxidizing method in which a de-oxidizing agent such as Al is added in the initial stage of de-oxidizing, or
There is a method of promoting floating separation of oxides by performing strong stirring in CAS treatment and RH treatment. On the other hand, for slag inclusions, a method of reducing the slag flowing out from the converter as much as possible by introducing a solidifying agent such as CaO before tapping the converter, or solidifying the slag on the pot with CaO or the like Therefore, there is a method of reducing the drawing into the molten steel.

[0006]

However, as in the proposal of the above-mentioned Japanese Laid-Open Patent Publication No. 1-180466, a method of using alumina-based inclusions as a low-melting slag-based inclusion of CaO-Al ₂ O ₃ is described below. If any of these substances remain in the molten steel, it will increase internal defects in the product. Moreover, the strong stirring in RH rather increases the inclusion of inclusions in the molten steel.

It is an object of the present invention to provide a method for producing molten steel for continuous casting, which reduces the amount of inclusions in a continuous casting process and leads to a good surface shape on a product plate and a reduction in internal defects. To do.

[0008]

The present invention is constructed to solve the above-mentioned problems, and by carrying out MgO blowing in the final stage of blowing in the converter, M in the converter slag is reduced.
The gO content is uniformly increased to 4% or more, the composition of the converter slag is controlled to the high melting point side, and the slag is prevented from flowing out during tapping by solidifying the slag by the effect of this high melting point. After reducing the amount of upper slag to 0.1% or less with respect to molten steel, and then uniformly spraying Al on the slag on the pan to modify FeO% in the slag to 3% or less, Ar from the bottom of the ladle
And deoxidizing under an inert gas while avoiding contact with slag, preferably by allowing a sedation time of at least 30 minutes or more after the deoxidizing treatment to reduce inclusions in the molten steel. The subject is a method for producing molten steel for continuous casting.

[0009]

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

The present inventors investigated the composition 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 abundance of spherical and amorphous morphologies in 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 at the stage of cast, and the composition is CaO: 40-60%, Al.
₂ O ₃ : Within the range of 40 to 60%. That is, inclusions (CaO <40%, Al
₂ O ₃ > 60%), it was found that 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 caught in molten steel, the components such as FeO, SiO ₂ , MnO in the slag are reduced by Al in the molten steel. , Surely CaO: 40-60%, Al
₂ O ₃ : The composition is in the range of 40 to 60%. Therefore,
It is necessary to control the composition of the slag on the pan to a composition that does not cause defects. This composition control tends to raise the melting point, but in order to uniformly control the composition toward the high melting point side, it is desirable to use strong stirring during blowing in the furnace.

FIG. 3 shows the uniformity of the composition of inclusions in molten steel when MgO was added as a high melting point side composition control agent during blowing and tapping. As you can see from the figure, M
It is preferable to add gO during blowing. Further, since the slag in the furnace is controlled to the high melting point side, the amount of slag flowing out from the converter is reduced, and the effect of significantly reducing the amount of slag on the ladle on the ladle is also recognized.

On the other hand, for alumina-based inclusions that cause sliver flaws, it is effective to reduce the oxidizing power in the slag. It is necessary to reduce the FeO% in the content to 3% or less. By the way, by investigating surface flaws (sliver flaws), it is found that these are alumina inclusions or C
It has been found that there is a characteristic that it is caused by a high melting point slag-based inclusion of aO <40% and Al ₂ O ₃ > 60%. That is, the measures against the low melting point slag-based inclusions described above lead to an increase in surface defects. Therefore, the present inventors have examined a deoxidation method that sufficiently separates the inclusions by floating.

FIG. 4 shows the RH deoxidation method and a simple molten steel treatment in which Ar is blown from the bottom of the ladle and deoxidation is performed under an inert gas while avoiding contact with slag. 118th Proceedings, 4th year (19
89), No. 2, p1235), and the amount and shape of inclusions after deoxidation treatment are compared. From these results, it is understood that the CAS treatment method greatly contributes to the floating separation of inclusions.

Next, examples of the present invention will be described.

[0017]

[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 containing 07 to 0.01% and Al: 0.04 to 0.06%. In a 350 ton converter, MgO blowing was performed in which MgO was added at the final stage of blowing, and the steel was adjusted to a predetermined carbon concentration and tapped. Al is sprayed on the ladle with a uniform spraying device to reduce the slag (FeO%) to 3% or less, and then Ar is blown from the bottom of the ladle to avoid contact with the slag and pure Al under inert gas.
Was deoxidized by adding 400 kg. After that, casting was performed by a curved continuous casting machine to form a slab cast having a thickness of 250 mm, and inclusions in the cast and the cold rolled sheet were investigated. Here, the presence / absence of MgO addition during blowing was examined, and a comparison was made between the CAS treatment method and the tap steel deoxidizing method and the RH deoxidizing method. Detailed conditions and results are shown in Table 1, Table 2 (continued-1 of Table 1) and Table 3 (continued-2 of Table 1).

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

Conventional Example 1 shows the current method, but the amount of slag inclusions is large because the slag flowing out from the converter cannot be suppressed. In addition, Comparative Examples 1 to 3 are Mg after blowing.
Although O is added and comparison is made by each deoxidizing method, as in the conventional example, the amount of slag-based inclusions increases because the slag flowing out from the converter cannot be completely suppressed. The amount of inclusions is CAS
It is slightly reduced by deoxidation, but the MgO content is small. Further, Comparative Examples 4 and 5 show the results when RH deoxidation and tapping deoxidation were carried out after MgO blowing at the end of blowing, but the MgO content in inclusions was relatively uniform. However, since the amount of inclusions is large, surface defects of the product are increasing.

[0022]

As described above in detail, according to the present invention, by performing MgO blowing at the end of blowing in the converter,
The MgO content in the converter slag is uniformly increased to 4% or more, the composition of the converter outflow slag is controlled to the high melting point side, and the slag is solidified by the effect of this high melting point, so that the slag outflow during tapping To reduce the amount of slag on the pan to 0.1% or less relative to the molten steel, and then uniformly spread Al on the slag on the pan to modify FeO% in the slag to 3% or less, By injecting Ar from the bottom of the pot and deoxidizing under inert gas while avoiding contact with slag, inclusions in molten steel can be rendered harmless and the amount of inclusions can be greatly reduced. Therefore, since the quality of the obtained product is good and the treatment is simple, a great cost effect can be obtained, and the present invention has a great effect on the industrial field.

[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 a composition of inclusions in a magnetic particle flaw detection portion.

FIG. 3 is a diagram showing the influence of the timing of MgO addition for composition control on the composition uniformity of inclusions.

FIG. 4 is a diagram showing a comparison of the amount of inclusions depending on the difference in deoxidation method.

Claims (1)

[Claims] 1. The MgO content in the converter slag is uniformly increased to 4% or more by performing MgO blowing in the final stage of the blowing in the converter, and the composition of the converter slag is controlled to the high melting point side. However, by solidifying the slag by the effect of this high melting point, the slag outflow during tapping is prevented, the amount of slag on the pan is reduced to 0.1% or less relative to the molten steel, and then it is evenly distributed on the slag on the pan. 3% FeO% in slag by spraying Al on
After reforming as follows, for continuous casting characterized by reducing the inclusions in the molten steel by blowing Ar from the bottom of the ladle and deoxidizing under inert gas while avoiding contact with slag. Manufacturing method of molten steel.