JPH1036909A - Production of highly clean steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of alumina cluster by controlling a primary Al concn. before adding Ti, in a deoxidizing treatment of molten steel. SOLUTION: A spheroidal inclusion of a low m.p. is produced in the molten steel after adding Ti by controlling Al quantity added according to soluble oxygen concn. in the molten steel before deoxidizing. This producing condition of the low m.p. inclusion is shown in the formula wherein, C(Al) is Al concn. (ppm) and C(O) is free oxygen concn. (ppm) in the molten steel before adding Al. Under such a condition, the deoxidizing is executed and, successively, Ti is added to cause reaction with remained soluble oxygen in the molten steel to form Ti base oxide, which is integrated with the remained Al2 O3 base inclusion in the molten steel form a low m.p. TiO2 -Al2 -O3 complex product. These are easily aggregated and integrated to be separated out of the system in shorter time than solid alumina cluster. As a result, the inclusion in the steel is reduced.

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 steel, and more particularly to a method for producing high-purity steel that reduces harmful alumina inclusions in molten steel.

[0002]

2. Description of the Related Art In the production of steel, it is known that nonmetallic inclusions generated by deoxidation treatment or secondary oxidation of molten steel have a significant adverse effect on quality. Therefore, a number of methods for reducing inclusions present in steel have been conventionally proposed. These methods are known as "high clean steel" (126th and 127th Nishiyama Memorial Technical Lecture, edited by The Iron and Steel Institute of Japan, November 1988).
Month).

[0003] These methods are roughly classified into a method of minimizing the number of inclusions and a method of rendering the inclusions harmless. The former example is a technique for preventing reoxidation by stirring in secondary refining or continuous casting. On page 11 of the same book, it is described that stirring promotes the floating and separation of deoxidized products. The page describes an inert gas seal in a tundish as a reoxidation prevention technique. An example of the latter is control of the composition of inclusions, and page 15 of the same book gives an example of a target inclusion composition.

In addition, Sakai et al. (Iron and Steel, 57th (197)
1) No. 13, p. 1863) evaluates the deoxidation behavior of various deoxidizers. Among them, as a kinetic feature of the cleaning of molten steel by composite deoxidization, promotion of agglomeration and coalescence by formation of a composite oxide with a low melting point is mentioned. The compound deoxidation is said to have a high cleaning effect on molten steel as compared to the single deoxidation.

[0005] However, for example, in the production of ultra-low carbon steel, Al kill is premised, and in the conventional deoxidation method, the deoxidation product is Al ₂ O _3. Can not expect.

[0006] In recent years, the quality requirements of consumers have become strict, and it is necessary to reduce the total amount of inclusions to the limit as described above. As described above, for example, this is a typical surface defect of a thin plate. It is known that sliver flaws are mainly caused by coarse alumina clusters of 500 μm or more captured near the surface layer of the slab. Therefore, it is necessary to reduce these coarse alumina clusters of 500 μm or more.

[0007]

As described above, if the number of coarse alumina clusters existing in molten steel can be reduced, not only the internal quality of the product but also the surface quality can be improved. The present invention is a method for producing high-purity steel,
It is an object of the present invention to provide a method for reducing the number of coarse inclusions, particularly alumina clusters.

[0008]

SUMMARY OF THE INVENTION The present invention has solved the above-mentioned problems, and its gist is that Al: 0.0% by weight.
In producing carbon steel having a composition of 1 to 0.1% and Ti: 0.01 to 0.1%, in the deoxidizing treatment step of molten steel, the primary Al concentration before adding Ti is expressed by the following formula. This is a method for producing high-purity steel, which is controlled within a range.

0.1 × C (O) <C (Al) <162.5+
0.375 × C (O) Here, C (Al): Al concentration (ppm) C (O): Free oxygen concentration in molten steel before addition of Al (ppm) Hereinafter, the present invention will be described in detail.

The present inventors conducted experiments in which the method of introducing Al in the deoxidation treatment was changed in various ways, and determined the conditions under which the composite deoxidation product of Al and Ti is formed during the process of molten steel during the composite deoxidation. investigated. As a result, if the amount of Al added before adding Ti is controlled so as to fall within the range of the present invention in accordance with the free oxygen concentration in the molten steel before deoxidation, it becomes spherical inclusions and floats out of the system.

The present inventors conducted deoxidation experiments with various amounts of Al and Ti added during the deoxidation process in order to reduce Al ₂ O ₃ inclusions in the steel. As a result, by controlling the amount of Al added according to the dissolved oxygen concentration in the molten steel before deoxidation, T
The following conditions were found under which low-melting spherical inclusions were formed in molten steel after the addition of i.

0.1 × C (O) <C (Al) <162.5+
0.375 × C (O) Here, C (Al): Al concentration (ppm) C (O): Free oxygen concentration in molten steel before adding Al (ppm) Further, under the condition that low melting point inclusions are formed. It has been found that when deoxidized, the amount of inclusions in the final product is reduced as compared to the conditions under which low melting point inclusions are not formed. This is because, when low-melting inclusions are generated, agglomeration and coalescence easily occur as compared with clusters formed by high-melting Al ₂ O _3, and more inclusions are separated out of the system due to a higher floating speed. is there.

In general, Al ₂ O ₃ easily forms alumina clusters, and once the alumina clusters are formed, the flow of molten steel inside the clusters is remarkably reduced. It is known that the flotation speed decreases as a result.

Therefore, in order to efficiently increase the cleanliness of the molten steel, when alumina clusters having a high density are formed, the stirring strength of the molten steel is increased to further increase the agglomeration rate, or forcedly blow Ar gas or the like. Must be combined with alumina clusters and separated by flotation.

However, if the stirring strength of the molten steel is excessively increased, the molten steel surface is disturbed due to the turbulence of the surface of the molten steel, such as reaction with the atmosphere or slag entrainment. Further, when the bubbles are blown, the above-described molten steel contamination may occur, and there is an upper limit to the amount of Ar blown.

The principle of the present invention can be considered as follows. When deoxidation is first performed with Al, an Al ₂ O ₃ -based oxide is generated in the molten steel. Next, when Ti is added, it reacts with the remaining dissolved oxygen in the molten steel to form a Ti-based oxide. Then, combined with Al ₂ O ₃ inclusions that existed in the molten steel to form a TiO ₂ -Al ₂ O ₃ composite deoxidation product having a low melting point. These tend to aggregate and coalesce and separate out of the system in a shorter time than solid alumina clusters. As a result, inclusions in the steel are reduced.

However, when the concentration of Al to be added is high, a sufficient amount of TiO ₂ is not generated due to low dissolved oxygen, and the inclusions in the molten steel are mainly Al ₂ O _3, and alumina Clusters are formed and the ascent rate is reduced. By controlling the amount of Al added within the range of the present invention, low melting point inclusions can be generated.

Next, the reason for determining the mathematical formulas shown in the formulas of the present invention will be described. The addition amount calculation formula shown in the present invention is a conditional expression obtained based on the above experimental results, and the upper limit is Al
Therefore, a large amount of Al ₂ O ₃ is generated, and low melting point inclusions are not generated even if Ti is added.

The lower limit is A which forms the lowest amount of Al ₂ O ₃ that can form a composite oxide by adding Ti.
1 is the amount added. When Al is completely added, solid TiO ₂ is generated after Ti is added, and the floating separation effect is reduced. Therefore, a small amount of Al ₂ O ₃ needs to be present before adding Ti. This amount is the amount of Al that combines with 1/10 of the oxygen in the molten steel.

The measurement of the free oxygen concentration in molten steel before the addition of Al can be performed by a commonly used oxygen sensor or the like.

[0021]

EXAMPLE In a high-frequency induction melting experiment, an Ar atmosphere,
1 kg of molten steel was melted in an MgO crucible, and after maintaining at 1570 ° C., predetermined amounts of Al and Ti were added, and immediately after the addition of Ti, a molten steel sample was taken. After holding for about 5 minutes after Ti addition, the power was turned off and air-cooled and solidified in the crucible. Free oxygen in the molten steel before and after the addition of Al was measured with an oxygen sensor. The obtained ingot was cut, the distribution of inclusions having a size of 10 μm or more was examined with an optical microscope, and the number of inclusions was determined.

Table 1 shows the components of each sample, the amount of deoxidizing element added, and the number of inclusions observed in the ingot. When the result deviates from the condition range of the present invention, the number of inclusions is larger than that of the present invention.

For confirmation, FIG. 1 shows the range in which spherical inclusions were observed in the sample collected after adding Ti. The spherical inclusions are generated within the condition range of the present invention, and it is understood that the cleanliness of the molten steel is improved by the generation of the spherical inclusions according to the concept of the present invention.

[0024]

[Table 1]

[0025]

As described above, the use of the method of the present invention makes it possible to reduce the amount of coarse alumina clusters which could not be reduced by the conventional method, and to reduce product defects. Therefore, it can be expected that the occurrence of surface defects in the product is reduced.

[Brief description of the drawings]

FIG. 1 is a view showing a range in which a spherical inclusion is observed in a sample collected after adding Ti.

Claims (1)

[Claims] 1. Al in weight%: 0.01-0.1%, T
i: When producing carbon steel having a composition of 0.01 to 0.1%, in the step of deoxidizing molten steel, the primary Al concentration before adding Ti is controlled within the range of the following formula. Manufacturing method of high clean steel. 0.1 × C (O) <C (Al) <162.5 + 0.375 ×
C (O) where C (Al): Al concentration (ppm) C (O): Free oxygen concentration (ppm) in molten steel before adding Al