JPH0610029A - Production of ultralow carbon steel - Google Patents

Abstract

PURPOSE:To remarkably quicken the decarburizing speed and to obtain a steel extremely low in carbon content by adding hydrogen into splash in a vacuum vessel at the latter stage of decarburization for which the carbon content in the molten steel becomes a specific value or lower at the tiame of executing the production of an ultralow carbon steel by an RH type vacuum degassing refining method. CONSTITUTION:The molten steel 5 is circulated between a ladle 4 and a vacuum 1 to decarburize the molten metal 5 in the ladle 4. Then, at the latter stage of the decarburization for which carbon content in the molten steel 5 becomes <=50ppm, one or more among the gaseous hydrogen, the hydrogen diluted with inert gas and solid metal hydride are added toward the splash 5a spattered from the molten steel 5 sucked up into the vacuum vessel 1 from a lance 7. By this method, an oxygen-enriched film or an iron oxide film generated on the splash 5a is reduced to promote the decarburizing reaction of the splash 5a. Thus, the ultralow carbon steel having <=20ppm carbon content is efficiently produced.

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 an ultra low carbon steel having a carbon content of 20 ppm or less by an RH type vacuum degassing refining method.

[0002]

2. Description of the Related Art The RH type vacuum degassing refining method is widely used as a method for industrially producing ultra-low carbon steel. In producing ultra-low carbon steel by such an RH-type vacuum degassing refining method, in recent years, higher quality and higher productivity have been required, and various methods have been proposed for that purpose.

For example, Japanese Patent Publication No. 62-6611 discloses RH.
When manufacturing ultra-low carbon steel by the vacuum degassing refining method, the dip tube connected to the bottom wall of the molten steel refining vacuum tank is formed into an elliptical cross-sectional shape, A method of increasing the cross-sectional area, thereby increasing the circulation amount of molten steel in the ladle with the vacuum chamber, and improving the removal rate of impurities in the steel (hereinafter referred to as prior art)
Is disclosed.

[0004]

However, according to the prior art, even if the removal rate of impurities can be improved to some extent, it cannot be expected to be greatly improved, and further, the refractory lining the dip tube is severely damaged. Therefore, the processing cost is increased. Therefore, in particular, a method for industrially and efficiently producing a very low carbon steel having a carbon content of 20 ppm or less has not been established yet.

Therefore, an object of the present invention is to solve the above-mentioned problems and to significantly speed up the decarburization rate in producing an ultra-low carbon steel by the RH type vacuum degassing refining method, whereby the carbon content is 20 ppm. It is to provide a method for efficiently producing the following ultra-low carbon steel.

[0006]

[Means for Solving the Problems] The present inventors have conducted extensive studies to solve the above-mentioned problems. As a result, when producing ultra-low carbon steel by the RH-type vacuum degassing refining method, in the latter stage of decarburization when the carbon content of the molten steel has dropped to 50 ppm or less, the splash scattered from the molten steel sucked into the vacuum tank If hydrogen is added to reduce the oxide film on the splash surface and the splash is used as a decarburization reaction site,
It has been found that an extremely low carbon steel having a carbon content of 20 ppm or less can be efficiently produced.

The present invention has been made on the basis of the above-mentioned findings, and an ascending pipe and a descending pipe vertically connected to the bottom wall of a molten steel refining vacuum chamber so as to project downward are described above. It is immersed in molten steel contained in a ladle located below the vacuum tank, and while depressurizing the inside of the vacuum tank, an inert gas is blown from the rising pipe to move the molten steel in the ladle to the rising pipe. Through the vacuum tank, degass the molten steel in the vacuum tank, and the molten steel sucked in the vacuum tank is returned to the ladle through the downcomer, the molten steel, In the method of circulating between the ladle and the vacuum tank, thus decarburizing the molten steel in the ladle, to produce an ultra-low carbon steel, the carbon content of the molten steel is
At the latter stage of decarburization of 50 ppm or less, toward the splash scattered from the molten steel sucked in the vacuum tank, gaseous hydrogen, hydrogen diluted with an inert gas, and
It is characterized by adding at least one of solid metal hydrides.

[0008]

When the ultra low carbon steel is produced by the RH type vacuum degassing refining method, the decarburization reaction occurs in the molten steel in which the inert gas is blown from the rising pipe, and the molten steel sucked up in the vacuum tank. Splashes scattered from the surface of the molten steel or the surface of the molten steel sucked up in the vacuum chamber can be considered.

Of the above, the decarburization reaction of the splashes scattered from the surface of the molten steel sucked up in the vacuum chamber is associated with oxygen in the steel during the early refining period while the carbon content in the steel is high. proceed. However, as refining progresses and the carbon content in the steel decreases, the oxygen content in the splash becomes much larger than the carbon content. Since oxygen has a surface adsorptive property, when the amount of oxygen in the splash increases, oxygen gathers on the splash surface to form an oxygen-enriched film or an iron oxide film. As a result, the decarburization reaction of molten steel is hindered.

Therefore, in the present invention, hydrogen is added toward the splashes scattered from the molten steel sucked up in the vacuum chamber in the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less. The hydrogen added in this way reduces the oxygen-enriched film and iron oxide film formed on the splash surface, and as a result, the decarburization reaction of the splash is promoted, which results in an extremely low carbon content of 20 ppm or less. Carbon steel can be efficiently manufactured.

The hydrogen added to the splashes scattered from the molten steel sucked up in the vacuum tank is gaseous hydrogen, gaseous hydrogen diluted with an inert gas such as argon, zirconium hydride, titanium hydride or the like. Solid metal hydrides, etc., and at least one of these
One is added to the splash.

FIG. 1 is a schematic vertical sectional view showing an example of an apparatus for carrying out the method of the present invention. As shown in FIG. 1, an ascending pipe 2 and a descending pipe 3 are vertically connected to a bottom wall 1a of a molten steel refining vacuum tank 1 so as to project downward. The lower part of each of the ascending pipe 2 and the descending pipe 3 is
Molten steel 5 contained in a ladle 4 located below the vacuum tank 1.
It is immersed in. An inert gas blowing pipe 6 is connected to the rising pipe 2. On one side wall 1b of the vacuum chamber 1, one of gaseous hydrogen, hydrogen diluted with an inert gas, and solid metal hydride toward the splash scattered from the molten steel 5 sucked up in the vacuum chamber 1 A lance 7 is provided for adding at least one. Reference numeral 8 is a conduit connected to a vacuum pump for reducing the pressure in the vacuum chamber 1.

While decompressing the inside of the vacuum chamber 1, an inert gas is blown through the blowing pipe 6 connected to the rising pipe 2 to move the molten steel 5 in the ladle 4 into the vacuum chamber 1 through the rising pipe 2. Molten steel 5 sucked up and sucked into the vacuum chamber 1
Is returned to the inside of the ladle 4 through the downcomer pipe 3, the molten steel 5 is circulated between the ladle 4 and the vacuum tank 1, and the molten steel 5 in the ladle 4 is decarburized. In the latter stage of decarburization when the carbon content of the molten steel 5 becomes 50 ppm or less, it is diluted with gaseous hydrogen and an inert gas from the lance 7 toward the splash 5a scattered from the molten steel 5 sucked up in the vacuum tank 1. Hydrogen and at least one of solid metal hydrides are added. As a result, as described above, the added hydrogen reduces the oxygen-enriched film and the iron oxide film formed on the surface of the splash 5a, thereby promoting the decarburization reaction of the splash 5a.

From the lance 7, it is necessary to add gaseous hydrogen or the like toward the splash 5a in the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less. The carbon content is
If gaseous hydrogen or the like is added to molten steel exceeding 50 ppm, the amount of dissolved oxygen will be reduced while the reaction force between carbon and oxygen is maintained to some extent, and as a result, the reaction force will decrease. There is a problem that the decarburization reaction becomes slow.

[0015]

EXAMPLES Next, the present invention will be described in more detail by way of examples in comparison with comparative examples. Molten steel processing capacity shown in Fig. 1
A 250 ton RH type vacuum degassing and refining device was used to decarburize molten steel having the chemical composition shown in Table 1 below.

The pressure in the vacuum chamber 1 was reduced to 1 torr or less, and argon gas was blown at a rate of 2000 Nl / min through a blow pipe 6 connected to the rising pipe 2 to melt the molten steel 5 in the ladle 4.
The molten steel 5 sucked up into the vacuum tank 1 through the rising pipe 2 and the molten steel 5 sucked up into the vacuum tank 1 through the descending pipe 3 is ladle 4
After returning to the inside, the molten steel 5 was circulated between the ladle 4 and the vacuum chamber 1 to decarburize the molten steel 5 in the ladle 4.

At the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less, hydrogen gas diluted with argon gas is directed from the lance 7 to the splash 5a scattered from the molten steel 5 sucked up in the vacuum chamber 1. , With a flow rate of 100 Nl / min so as not to directly hit the molten steel.

FIG. 2 is a graph showing the carbon content in the molten steel in relation to the treatment time when the hydrogen gas diluted with the argon gas is sprayed in this manner. In FIG. 2, the vertical axis represents the carbon content in the molten steel, and the horizontal axis represents the treatment time. Then, the solid line shows the carbon content that changes with time when treated by the method of the present invention, and the dotted line shows the carbon content that changes with time in the case of the conventional method in which hydrogen is not blown. Indicates.

As is apparent from FIG. 2, according to the method of the present invention, the decarburization reaction of the molten steel has a carbon content of 50 pp.
Even after it became less than m, it proceeded without stagnation, its decarburization rate was fast, and the carbon content of molten steel was 10ppm in 20 minutes after the treatment.
It could be reduced to below.

[0020]

As described above, according to the method of the present invention, the decarburization rate can be remarkably increased when the ultra low carbon steel is produced by the RH type vacuum degassing refining method. Industrially useful effects capable of efficiently producing an ultra-low carbon steel having a content of 20 ppm or less are brought about.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing an example of an apparatus for carrying out the method of the present invention.

FIG. 2 is a graph showing the relationship between the carbon content in molten steel and the treatment time when decarburizing the molten steel by the method of the present invention.

[Explanation of symbols]

 1 vacuum tank, 7 lances, 2 risers, 8 conduits. 3 downcomers, 4 ladle, 5 molten steel, 5a splash, 6 gas injection pipe,

Claims (1)

[Claims] 1. An ascending pipe and a descending pipe vertically connected to a bottom wall of a molten steel refining vacuum tank so as to project downward are housed in a ladle located below the vacuum tank. Immersed in molten steel, while depressurizing the inside of the vacuum tank, blowing an inert gas from the rising pipe, sucking the molten steel in the ladle into the vacuum tank through the rising pipe, the molten steel in the vacuum tank Degassed inside, and the molten steel sucked up in the vacuum tank is returned to the ladle through the downcomer pipe, the molten steel is circulated between the ladle and the vacuum tank, Thus, in the method of decarburizing the molten steel in the ladle to produce an ultra-low carbon steel, in the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less, the molten steel sucked into the vacuum tank Hydrogen and inert gas toward the splashes scattered from Therefore diluted hydrogen, and at least one is characterized by adding, method for producing ultra low carbon steel of the solid metal hydride.