JP2013079413A - Method for producing high-nitrogen steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress variation in nitrogen content rate to a low level, and also to improve an operation efficiency.SOLUTION: A method for producing a high-nitrogen steel refines a molten steel 3 that has been refined in a refining vessel 2, further by a circulation degassing process. The method includes a step of loading a ferrosilicon nitride 1 with the largest grain size of 1 mm or less into the refining vessel 2, prior to the circulation degassing process.

Description

  The present invention relates to a method for producing high nitrogen steel, and more particularly to a method for producing high nitrogen steel having a small variation in nitrogen content.

  As a manufacturing method that can suppress the variation in the nitrogen content in high nitrogen steel, for example, Patent Document 1 proposes to add silicon iron nitride powder during the refining stage in the refining vessel or during the cyclic degassing process. Has been.

JP-A-7-70691

  However, even with the above proposed method of adding silicon iron nitride, the variation in nitrogen content is still not sufficiently suppressed, and the amount of addition is limited because a large amount of slag forming occurs when silicon iron nitride is added. Further, there is a problem that the operation efficiency is deteriorated due to poor slag removal during the subsequent circulation degassing treatment.

  Then, this invention solves such a subject and it aims at providing the manufacturing method of the high nitrogen steel which can improve the operation efficiency while being able to suppress the dispersion | variation in nitrogen content rate small. .

  As a result of various experiments, the inventors have found that the conventional method of adding silicon iron nitride powder causes a variation in nitrogen content and a large amount of foaming. The present invention has been devised based on the idea that the diameter is as coarse as 3 mm to 10 mm.

  That is, the present invention relates to a method for producing high nitrogen steel in which the molten steel (3) refined in the refining vessel (2) is further refined by circulating degassing treatment, and the refining vessel (2) is provided prior to the circulating degassing treatment. Is a method for producing high nitrogen steel, characterized in that silicon iron nitride (1) having a maximum particle size of 1 mm or less is introduced into the steel.

  According to the present invention, silicon iron nitride having a maximum particle size of 1 mm or less is retained in the slag that covers the surface of the molten steel in the refining vessel and does not reach the molten steel, and the formation of forming occurs due to the rapid penetration into the slag. Can be kept small. Therefore, the slag that has entered the degassing tank during the degassing process quickly returns to the refining vessel, and only the molten steel is exposed to the degassing atmosphere, and unnecessary gas components and inclusions are removed from the molten steel. According to this, since the nitrogen content in the slag is maintained at a high level and the fluctuation is suppressed, it is possible to obtain a high nitrogen steel with a small variation in the nitrogen content, and the forming is suppressed and the operation efficiency is improved. .

  In addition, the code | symbol in the said parenthesis shows the correspondence with the specific means as described in embodiment mentioned later.

  As mentioned above, according to the manufacturing method of the high nitrogen steel of this invention, the dispersion | variation in nitrogen content rate can be suppressed small, and operation efficiency can also be improved.

It is a general | schematic vertical sectional view of the ladle explaining the process of this invention method. It is the graph which compared the nitrogen concentration in slag with the method of this invention, and the conventional method. It is a general | schematic vertical sectional view of the degassing tank explaining the process of this invention method. It is the graph which compared the time to slug removal by the method of the present invention and the conventional method. It is a general | schematic vertical sectional view of the ladle explaining the process of this invention method. It is the graph which compared the nitrogen concentration range of high nitrogen steel with the method of this invention, and the conventional method. It is a general | schematic vertical sectional view of the ladle explaining the process of the conventional method. It is a general | schematic vertical sectional view of the degassing tank explaining the process of the conventional method.

  The embodiment described below is merely an example, and various design improvements made by those skilled in the art without departing from the gist of the present invention are also included in the scope of the present invention.

  In the method of the present invention, silicon iron nitride (N-FSi) powder is put into a ladle serving as a refining vessel at the final stage of ladle refining prior to circulation degassing (RH degassing). In this case, the maximum particle size of the powder is 1 mm or less. Such a powder can be obtained by pulverizing silicon nitride iron and then removing the powder having a particle size larger than 1 mm with a sieve of a specific mesh. As silicon iron nitride, for example, a composite refractory raw material containing 75% to 80% silicon nitride (Si3N4) component and containing 12% to 17% free iron can be used.

  As shown in FIG. 1, when silicon iron nitride 1 having a maximum particle size of 1 mm or less is put into ladle 2, it is retained in slag 4 covering the surface of molten steel 3 in ladle 2, and in molten steel 3 Not reach. And since melt | dissolution to the slag 4 is quick, generation | occurrence | production of slag forming is suppressed small. That is, as shown by the line X in FIG. 2, when the silicon iron nitride powder of the present invention having a maximum particle size of 1 mm or less is added, the nitrogen concentration in the slag increases smoothly with the passage of time after the addition. On the other hand, when silicon iron nitride powder having a conventional powder particle size of 3 mm to 10 mm is added, as shown by line Y in FIG. Does not rise. This is probably because the injected silicon iron nitride reaches the molten steel. In addition, the addition amount of the silicon iron nitride powder in FIG. 2 is 15 kg.

  After the ladle refining is completed, the process proceeds to the circulating degassing process as shown in FIG. Since the forming is suppressed, the slag 4 that has entered the degassing tank 5 is circulated as shown by the arrow in the figure and quickly returned to the ladle 2 (slag removal), and only the molten steel 3 is degassed. Unnecessary gas components and inclusions are removed from the molten steel 3 by being exposed to the atmosphere. That is, as shown in FIG. 4, when the silicon iron nitride powder of the present invention having a maximum particle size of 1 mm or less is added (black circle mark in the figure), the time until slag removal is as short as 2 to 7 minutes. On the other hand, when a conventional silicon iron nitride powder having a powder particle size of 3 mm to 10 mm is added (white square marks in the figure), it takes about 6 to 12 minutes. The horizontal axis in FIG. 4 indicates the number of ch (charges) when the same steel type is continuously processed with the addition amount of silicon iron nitride powder being 60 kg / ch. In the conventional silicon iron nitride powder, the degassing tank is shown. Since the slag gradually adheres inside and the number of channels increases, the time until slag removal becomes longer, whereas the silicon iron nitride powder of the present invention is not affected by slag adhesion, and the time until slag removal Keep short regardless of number.

  After completion of the circulation degassing process, as shown in FIG. 5, the nitrogen (N) content from the silicon nitride iron retained in the slag 4 is transferred into the molten steel 3 by the slag metal interface reaction. In the present invention, since the nitrogen concentration in the slag 4 can be kept high as described above, the nitrogen content is efficiently transferred into the molten steel 3.

  High nitrogen steel with small variation in nitrogen content can be obtained by the above process. This is shown in FIG. FIG. 6 shows the nitrogen concentration range of the high nitrogen steel obtained in the above process. When the silicon iron nitride powder of the present invention having a maximum particle size of 1 mm or less is used, the nitrogen concentration of the high nitrogen steel is 0.005%. Whereas the conventional silicon iron nitride powder having a particle diameter of 3 mm to 10 mm is used, the nitrogen concentration range of the high nitrogen steel is 0.001% to 0.007. %. In this case, the amount of silicon iron nitride powder added is 60 kg.

  The cause is considered as follows. That is, when the powder particle size is large, as shown in FIG. 7, when the silicon nitride iron powder 1 ′ is put into the ladle 2, the silicon nitride iron powder 1 ′ does not stay in the slag 4 but enters the molten steel 3. And nitrogen is degassed during the degassing process. In addition, since the particle size is large, forming continues after the addition and occurs in large quantities. In the subsequent circulation degassing treatment (FIG. 8), forming by unreacted silicon iron nitride 1 ′ occurs in the slag 4 that has entered the degassing tank 5, and the slag 4 is not easily removed. Nitrogen dissolved in the gas is degassed into the degas atmosphere. As a result, the nitrogen content in the molten steel 3 after the circulation degassing treatment and the nitrogen content of the silicon iron nitride retained in the slag 4 vary greatly, and the nitrogen concentration range of the resulting high nitrogen steel varies greatly. .

  In the present invention, the variation in the nitrogen concentration range of the high nitrogen steel can be suppressed, and the forming at the time of addition of silicon iron nitride can be suppressed small. Therefore, the addition amount of silicon iron nitride is increased to further increase the nitrogen concentration. High nitrogen steel can be obtained, and since the slag can be easily removed in the circulating degassing process, the operation efficiency can be increased.

  1 ... silicon iron nitride, 2 ... ladle (smelting vessel), 3 ... molten steel.

Claims (1)

In a method for producing high nitrogen steel in which molten steel refined in a refining vessel is further refined by circulating degassing treatment, iron iron nitride having a maximum particle size of 1 mm or less is introduced into the refining vessel prior to the circulating degassing treatment. A method for producing high nitrogen steel characterized by the following.

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