JPH07207326A - Production of molten steel for continuous casting - Google Patents

Abstract

PURPOSE:To reduce inclusion in molten steel in a continuous casting process. CONSTITUTION:In a converter, slag is solidified by charging CaO and the flow- out of the slag is prevented during steel tapping and the slag quantity on a ladle is reduced to <=1wt.% to the molten steel. Thereafter, Al is uniformly scattered on the slug on the ladle, and after reducing FeO% in the slag to <=3%, Al is charged and oxygen blowing is executed. Thereafter, Ar is blown from the ladle bottom part and deoxidation is executed under the inert gas atmosphere while avoiding the contact with the slag to reduce the inclusion. By this method, since the quality of the molten steel can remarkably be improved, the extremely 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 the following two types of inclusions. 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 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 configured to solve the above-mentioned problems, and in a converter, Ca
By adding O, the slag is solidified, the outflow of the converter slag during tapping is prevented, and the amount of slag on the pan is reduced to 0.1% or less by weight ratio to the molten steel. After uniformly dispersing Al into the slag to reduce FeO% in the slag to 3% or less, Al is introduced into the molten steel to blow oxygen on the surface of the molten steel, and then Ar is blown from the bottom of the ladle to form slag. SUMMARY OF THE INVENTION A gist is a method for producing molten steel for continuous casting, which is characterized in that inclusions in the molten steel are reduced by performing deoxidation under an inert gas while avoiding contact with the molten steel.

[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.

By the way, the present inventors have conducted an investigation of the composition of inclusions after heating Al and found that the composition of the produced inclusions is an Al-Mn-Si-O type composite oxide. . As can be seen from the composition, this inclusion is a low-melting inclusion at the time of formation, and is a liquid spherical inclusion in the molten steel. The present inventors investigated the composition and amount of inclusions in molten steel and the composition of slag on the pot depending on whether or not Al heating was performed, and compared the two. FIG. 3 shows the result. Regarding the composition of inclusions in molten steel,
After that, it is considered that the components such as SiO ₂ and MnO are reduced by Al in the molten steel, and it is shown in the binary system of CaO and Al ₂ O ₃ . Table 1 shows the composition of slag on the pan.

[0014]

[Table 1]

As can be seen from these results, when Al heating is carried out, most of them are CaO: 40-60%, Al
₂ O ₃ : deviates from the composition within the range of 40 to 60%, and Al ₂ O
_{3 It has a} rich composition. That is, CaO: 40-
60%, Al ₂ O ₃ : A low-melting point suspension slag in the range of 40 to 60% and a composite oxide due to Al heating are aggregated with each other to form a composition on the Al ₂ O ₃ rich side. It is thought that it has become.

On the other hand, although the amount of inclusions is increased by the Al heating, the total amount of inclusions only tends to increase slightly. This is considered to have contributed to the acceleration of the floating of the inclusions by agglomeration, in addition to the control of the composition of the inclusions to the high melting point side as described above. by the way,
Although this method slightly contributes to the reduction of inclusions,
It means that the low melting point inclusions were replaced with the high melting point inclusions,
On the contrary, it leads to worsening surface defects. Therefore, the present inventors have examined a deoxidation method that sufficiently separates the inclusions by floating.

FIG. 4 shows a tapping deoxidizing method and an RH deoxidizing method, and further, a simple molten steel treatment in which Ar is blown from the bottom of a ladle and deoxidizing is performed under an inert gas while avoiding contact with slag (for example, , "Materials and Processes-The Iron and Steel Institute of Japan, 118th Lecture Collection," 4th year (1989), No. 2, p123.
5) is a comparison of the amount and shape of inclusions after deoxidation treatment. From these results, it became clear that the CAS treatment method greatly contributes to the floating separation of inclusions.

Next, examples of the present invention will be described.

[0019]

[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, after blowing, the steel was adjusted to a predetermined carbon concentration and tapped. Al is sprayed on the ladle with a uniform spraying device, and (F
eO%) is reduced to 3% or less, and then Al is heated, then Ar is blown from the bottom of the ladle and 400 kg of pure Al is charged under inert gas while avoiding contact with the slag to deoxidize it. went. After that, cast with a curved continuous casting machine, 250
mm slab slabs were produced. Here, the CAS treatment method was compared with the molten steel treated by the conventional steel deoxidizing and RH deoxidizing methods. Detailed conditions and results are shown in Tables 2 and 3.
(Continued-1 of Table 2) and Table 4 (Continued-2 of Table 2)
Shown in.

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

In Comparative Example 1, the outflow of the converter slag could not be suppressed, and the defects of the internal product increased. Further, in Comparative Examples 2 and 3, it can be seen that the surface defects are increased when the slag modification is insufficient or the modification cannot be performed uniformly. Furthermore, in Comparative Examples 4 and 5, the deoxidation method was changed, but the effect of floating inclusions was not recognized as compared with CAS. Finally, Comparative Examples 6 to 8 show the cases where the deoxidation method was changed without performing Al heating, but no reduction in slag-based inclusions was observed.

[0024]

As described above in detail, according to the present invention, the slag is solidified by introducing CaO in the converter to prevent the converter slag from flowing out during tapping to prevent the amount of slag on the pot. Is reduced to 0.1% or less by weight ratio with respect to the molten steel, and then Al is evenly sprayed on the slag on the pan to make Fe in the slag
After reducing the O% to 3% or less, Al is heated, and Ar is blown from the bottom of the ladle, and deoxidation is performed under an inert gas while avoiding contact with the slag to make the inclusion composition harmless. It is possible to reduce the amount of inclusions.
Therefore, since the product quality is greatly improved, 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 a comparison between the composition and amount of inclusions with and without Al heating.

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

Claims (1)

[Claims] 1. In the converter, CaO is charged to solidify the slag to prevent the converter slag from flowing out during tapping, and the amount of slag on the pan is 0.1 to the molten steel in a weight ratio.
% Or less, and then Al is evenly sprayed on the slag on the pan to reduce FeO% in the slag to 3% or less, and then Al is injected into the molten steel to blow oxygen on the molten steel surface, and thereafter, A method for producing molten steel for continuous casting, characterized in that inclusions in molten steel are reduced by blowing Ar from the bottom of a ladle and performing deoxidation under an inert gas while avoiding contact with slag.