JPS61223121A - Method for refining low nitrogen steel - Google Patents

Abstract

PURPOSE:To efficiently denitrify molten steel at a low cost by blowing hydrogen-base gas into the molten steel to regulate the amount of hydrogen in the molten steel to a specified value and by vacuum-degassing the molten steel. CONSTITUTION:Molten steel 1 is poured into a molten metal vessel 2 such as a ladle, the immersion pipes 3 of a reflux system vacuum degassing apparatus 6 are put in the molten steel 1, and hydrogen-base gas 5 is blown into the molten steel 1 through porous plugs 4 to regulate the amount of hydrogen in the molten steel 1 to 5-25ppm. When the molten steel 1 having the increased hydrogen content is introduced into the apparatus 6 through one of the pipes 3, the hydrogen in the introduced molten steel 1 generates fine bubbles 7. These bubbles capture nitrogen in the molten steel 1 and are exhausted to a vacuum exhaust system.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for producing low nitrogen steel.

(Prior art) Conventionally, in order to produce low-nitrogen steel, a method was used in which molten steel was blown into low-nitrogen steel in a converter, etc., and the composition and temperature were adjusted while shielding it from the atmosphere until it was cast. was. (For example, Japanese Patent Application Laid-Open No. 58-197209). In this case, in order to prevent an increase in the amount of nitrogen due to nitrification from the ferroalloy, it is necessary to use an expensive ultra-low nitrogen ferroalloy. , which caused an increase in the manufacturing cost of molten steel. For this reason, as a method for denitrifying molten steel after adding alloys, a large amount of argon gas blowing method has been developed in the reflux vacuum gas method. (Japanese Unexamined Patent Publication No. 58-213819).

However, this method is not preferable because a large amount of gas is used, which causes a large drop in the temperature of the molten steel, and there is a lot of molten steel scattering in the vacuum chamber, causing operational problems due to metal adhesion.

(Problems to be Solved by the Invention) An object of the present invention is to solve the above-mentioned problems and provide a low nitrogen steel melting method that can perform molten steel denitrification treatment inexpensively and efficiently. It is something.

(Means for Solving the Problems) The gist of the present invention is that when molten steel is subjected to vacuum degassing treatment, a hydrogen-based gas is blown into the molten steel to reduce the amount of hydrogen in the molten steel to 5 ppm or more. , 25 ppm or less, the molten steel is placed under vacuum to dehydrogenate and denitrify.

Generally, when denitrifying molten steel under vacuum, an inert gas such as argon gas is blown into the molten steel, nitrogen in the molten steel is moved into the inert gas, and the nitrogen gas is discharged out of the molten steel. In this case, in order to increase the denitrification rate, it is preferable to increase the surface area of the blown gas bubbles and to stir the molten steel side to increase the nitrogen diffusion rate. As in the present invention,
When hydrogen-based gas is blown into molten steel, the hydrogen content in the molten steel increases. In this case, if the hydrogen content in the molten steel is less than 5 ppm, the denitrification efficiency will not improve;
If it exceeds ppm, hydrogen gas will not be completely dissolved in the molten steel and will be directly emitted into the atmosphere, reducing the efficiency of the blown gas.
Undesirable. Therefore, the hydrogen content in molten steel is 5 ppm.
As mentioned above, in order to keep the amount of hydrogen at 25 ppm or less, it is desirable to adjust the amount of blown gas while analyzing the amount of hydrogen in the molten steel. When molten steel with increased hydrogen content is led into the vacuum chamber through the immersion tube of a recirculation vacuum degassing device, the hydrogen dissolved in the molten steel becomes supersaturated in the vacuum chamber under reduced pressure, and hydrogen gas Bubbles are generated, and nitrogen in the molten steel is captured in the hydrogen gas bubbles while being guided to the vacuum evacuation system. Very busy,
Hydrogen has a large diffusion coefficient in molten steel and is an element suitable for generating gas bubbles. In this way, the bubbles generated directly from molten steel are fine bubbles compared to argon gas, which is normally used as a reflux gas, and the specific surface area of the gas bubbles is large, so the amount of gas bubbles per unit gas amount is small. This has the advantage of increasing the reaction interfacial area of 2. Furthermore, when bubbles are generated directly from molten steel, the gas-molten steel interface moves significantly due to the generation of gas. When the gas-molten steel interface moves in this way, relatively speaking, nitrogen in the molten steel moves toward the gas-molten steel interface, resulting in an increase in the diffusion rate of nitrogen atoms in the molten steel. When molten steel with increased hydrogen content is denitrified in this manner, the denitrification rate is improved from the two viewpoints of increasing the bubble interface and increasing the diffusion rate of nitrogen in the molten steel.

Next, an example of the method of the present invention will be described. As shown in Fig. 1, molten steel melted in a melting furnace such as a converter or an electric furnace is transferred to a molten metal container 2 such as a ladle, and the immersion tube 3 of a circulation type vacuum degassing device 6 is inserted into the molten steel. Soak in. Subsequently, hydrogen-based gas 5 is blown into the molten steel t in the molten metal container through the porous plug 4 disposed at the bottom of the molten metal container to increase the amount of hydrogen in the molten steel. When molten steel with increased hydrogen content in this way is introduced into the vacuum chamber through the immersion in the reflux type vacuum degassing @#, the hydrogen in the molten steel generates fine bubbles 7 in the vacuum chamber, and these The microbubbles capture nitrogen in the molten steel and are guided to the vacuum exhaust system. In this way, nitrogen in the molten steel is efficiently degassed. In addition, as a method of blowing the hydrogen-based gas, as shown in FIG. 2, it may be blown into the molten steel through a lance.

Further, as a degassing device, a suction type vacuum degassing device at as shown in FIG. 2 can also be used. Molten steel 1 melted in a melting furnace such as a converter or an electric furnace is transferred to a molten metal container 2 such as a ladle, and a suction pipe 10 of a suction type vacuum degassing device 9 is immersed in the molten steel. Subsequently, a hydrogen-based gas 5f is blown through the entire molten steel injection valve 8 in the molten metal container to increase the water St in the molten steel. When molten steel with increased hydrogen content is introduced into the vacuum chamber through the suction pipe of the suction type vacuum degassing device, the hydrogen in the molten steel generates fine bubbles 7 in the vacuum chamber, and these fine bubbles The nitrogen in the molten steel is captured and guided to the vacuum evacuation system.□In this way, as a degassing device, either a circulation type vacuum degassing device or a suction type vacuum degassing device can be used.

After denitrification, if the addition of hydrogen-based gas is stopped and normal degassing operation is performed, the previously blown hydrogen-based gas will be discharged to the vacuum exhaust system by the mechanism described above during degassing operation. The amount remaining in the molten steel after vacuum treatment is about 1.5 ppm or less, and it is possible to prevent the quality from becoming particularly rough.

(5J!Example) Table IK shows an example. Example 1 is a case in which hydrogen gas is injected from a porous plug at a rate of 4 Nm"/min, and Example 2 is a case where hydrogen gas is blown in at a rate of 4 Nm"/min from a porous plug.
In Example 3, methane gas was blown into the porous plug at a rate of 2 NmS/min.
Example 4 uses acetylene gas at 4 Nm”/m from a lance.
When injected, the hydrogen content in the molten steel during treatment is 5
The amount was blown while adjusting the amount to be between ppm and 25 ppm. Moreover, Comparative Example 1 is a case where argon gas was blown in instead of hydrogen-based gas. All of the slag pots used are aluminum-silicon killed steel melted in a converter and whose composition is adjusted in a ladle.

In all of Examples 1 to 4, the amount of nitrogen in the molten steel could be denitrified to 10 PPm or less. On the other hand, in the case of Comparative Example 1, the denitrification was only up to 25 ppm, and compared to the Example,
A sufficient denitrification effect could not be obtained.

As described above, by applying the method of the present invention to the denitrification treatment of molten steel, it has become possible to easily produce low nitrogen steel.

(Effects of the Invention) According to the present invention, compared to the conventional melting method for low nitrogen steel,
There is almost no modification or new installation of melting furnace equipment, equipment for transferring from the melting furnace to the molten metal container, degassing equipment, etc., and low-nitrogen steel can be melted simply by installing hydrogen-based gas blowing equipment. Furthermore, it is also possible to prevent operational problems caused by metallurgy in K, such as large amounts of argon injection, which is a conventional method for producing low nitrogen steel. In addition, the nitrogen content in molten steel was reduced to 10 ppm within the normal degassing treatment time of 20 minutes.
It has become possible to produce the following unprecedented low-nitrogen steel.

As described above, according to the present invention, molten steel can be denitrified easily and quickly compared to conventional methods.

Further, excellent effects such as accurate denitrification on an industrial scale can be obtained.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a method for denitrification using a reflow type vacuum degassing device while blowing hydrogen-based gas through a porous plug at the bottom of the ladle, and Figure 2 shows a method for denitrification by blowing hydrogen-based gas through a lance. This is a schematic diagram of a method for denitrification using a vacuum degassing device. Figure 1

Claims (1)

[Claims] Blow hydrogen gas into the molten steel to reduce the amount of hydrogen in the molten steel to 5p
A method for producing low nitrogen steel, which comprises adjusting the nitrogen content to pm or more and 25 ppm or less, and then subjecting it to vacuum degassing treatment.