JP3407326B2 - Manufacturing method of low nitrogen steel - Google Patents

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 low nitrogen steel by spraying oxidant powder.

[0002]

2. Description of the Related Art Generally, nitrogen in carbon steel is one of the elements that form a nitride with Al, Ti, etc. and have a great influence on the refinement of crystal grains or other characteristics. For example, Al
There are cold rolled steels of Al-killed steel that are annealed by generating a certain amount of N 2, and cold rolled steels that are continuously annealed by reducing [N] in molten steel as much as possible.

Examples of methods for adding nitrogen to these carbon steels include addition of nitrogen-containing alloy iron, CaCN _2, etc., or N ₂ bubbling method. On the other hand, however, degassing treatment for the purpose of denitrifying molten steel is not actively carried out, and RH equipment, VOD equipment, etc. for decarburization of crude molten steel produced in a steelmaking furnace are used. The degassing under reduced pressure was carried out, and denitrification was only confirmed as a side effect.

That is, in the case of conventional low carbon Al killed steel, in order to reduce [C] in molten steel melted in a steelmaking furnace such as a converter, vacuum degassing treatment is performed using a vacuum treatment equipment such as an RH device. Nitrogen is necessary to promote AlN fine precipitation during box annealing of the cold-rolled steel sheet in the post-process, and was controlled within a certain concentration range.

However, due to the progress of rolling technology, the continuous annealing method has recently been adopted, which is a rapid heating,
Since it is a quenching process, AlN is unlikely to precipitate, and the Rankford value (r value) as an index of workability cannot be expected to improve.
For this reason, the amount of residual solid solution nitrogen is increased and strain aging is likely to occur.

In order to improve this, it is effective to reduce the nitrogen concentration in the product of the rolling mill to 20 ppm or less,
For this reason, it is highly desirable to anticipate the pickup of nitrogen in the molten steel during continuous casting or ingot making, which is the upper process of rolling, and stabilize [N] at each charge stable at 10 ppm or less at the end of RH treatment. Was there. This is, for example, during continuous casting,
Although there are variations depending on the degree of sealing in the process between the ladle and the tundish, the nitrogen pickup is at least about 2 to 3 ppm even if the sealing is performed accurately.

On the other hand, Japanese Patent Application Laid-Open No. 60-184618 discloses a method of accelerating denitrification by upwardly blowing an oxidizer powder under reduced pressure. However, there was no clear regulation on the powder top blowing condition, and the denitrification promoting effect was not remarkable in some cases.

This is because the method disclosed in Japanese Patent Application Laid-Open No. 60-184618 does not clarify the conditions under which the powder penetrates the molten steel, and as disclosed in the examples,
In all cases, the powder supply rate, particle size, and lance height are limited to a narrow range, and it cannot be said that they are sufficient in terms of supply conditions for obtaining a stable denitrification promoting effect.

[0009]

DISCLOSURE OF THE INVENTION An object of the present invention is to effectively accelerate the denitrification reaction in the denitrification of molten steel by spraying the powder under reduced pressure to stabilize the nitrogen level of the steel at a low level, and It is to reduce the body supply to 2/3 to 1/2 of the conventional level.

[0010]

DISCLOSURE OF THE INVENTION The present inventors have defined as refining conditions for powder to sufficiently penetrate into molten steel under vacuum.
The inventors have completed the present invention by knowing that it is necessary to quantitatively determine the particle diameter of powder, the feed rate of powder, and the distance between the lance for blowing powder and the surface of molten steel in various refining apparatuses.

In the refining method of decarburizing and denitrifying by spraying an oxidizer powder onto molten steel under reduced pressure, the present invention has a particle size of 10 to 200 μm. This is a method for producing low-nitrogen steel, characterized in that the powder supply rate is 0.1 to 1.0 kg / min · ton.

According to a preferred embodiment of the present invention, when a denitrification process is performed using a vacuum decarburization furnace (VOD furnace) as a so-called vacuum refining furnace, or a cylindrical dipping tube is placed in molten steel in a ladle. When immersing, decompressing the inside of the dipping pipe to suck molten steel into the dipping pipe and performing denitrification treatment, dipping from the tuyere located at the bottom of the ladle or the lance located below the dipping pipe The inert gas is blown toward the inside of the tube, and the oxidizer powder with a particle size of 10 to 200 μm is supplied from the powder injection lance with a distance of 0.5 to 2.0 m from the molten steel surface inside the immersion tube. 0.
Top-spray at 1 to 1.0 kg / min ton.

According to still another aspect, the reflux type degassing device is immersed in the molten steel in the ladle, the molten steel is refluxed in the refluxing type degassing device, and the molten steel in the refluxing type degassing device is refluxed. Oxidizer powder with a particle size of 10 to 200 μm is supplied from a powder injection lance with a distance from the surface of 1.0 to 3.0 m at a powder supply rate of 0.1 to 1.0 k
Top blow with g / min ton.

[0014]

Next, the operation of the present invention will be described more specifically. In the present invention, when the molten steel to be treated,
The amount of carbon is not particularly limited. For example, [C] = 0.04%
It can be: In the upper blowing of the powder in the present invention, a blowing condition for allowing the oxidant powder to sufficiently penetrate into molten steel under vacuum is shown. Here, under vacuum is 20To
It shall indicate the state below rr.

First, the particle size of the powder is suitably 10 to 200 μm. If the powder is less than 10 μm, the powder does not soar into the vacuum chamber and penetrate into the molten steel, making it difficult to stably supply the powder into the molten steel. On the other hand, in the case of the powder having a particle size of more than 200 μm, the reaction interfacial area of the entire powder is remarkably reduced, so that the reaction promoting effect is significantly reduced. Preferably 50-100 μ
m.

Next, the powder feeding rate is 0.1 to 1.0 k
g / min-t is appropriate. If it is less than 0.1 kg / min · t, the denitrification reaction rate of molten steel is slow and the treatment time increases, which is inefficient. On the other hand, above 1.0 kg / min · t, it was observed that the reaction could not cope with the powder supply rate, and unreacted powder remained or accumulated on the molten steel surface. As for the distance between the lance for blowing the powder and the surface of the molten steel, the following findings were obtained as a result of various experiments.

In order to sufficiently infiltrate the above-mentioned oxidizer powder into the molten steel under a vacuum of 20 Torr or less, in a device for making the entire ladle containing the molten steel into a vacuum atmosphere, for example, in a VOD furnace, the powder is The distance between the blowing lance and the surface of the molten steel is 0.5 to 2 m. For example, as proposed in JP-A-1-92314, an inert gas is introduced from the lance or the bottom tuyere while the immersion pipe is evacuated to a vacuum. In the ladle refining device characterized by blowing air, the distance between the lance that blows the powder and the molten steel surface is 0.5 to 2 m. In the reflux type degassing device, the distance between the lance that blows the powder and the molten steel surface is 1 to 1. It is necessary to make it 3 m.

Here, the lower limit between the molten metal surfaces of the lance is the minimum value at which the lance is not melted by the molten steel splash, and the upper limit is
It is the maximum value that powder can penetrate into molten steel. All of these are empirical values obtained as a result of various operations.

FIG. 1 shows a vacuum refining furnace, for example, a vacuum decarburizing furnace (V
(OD furnace) is a schematic explanatory diagram of the implementation situation of the present invention,
In the figure, bottom-blown Ar gas is blown into the molten steel in the ladle 12 placed in the vacuum decarburizing furnace 10 from the bottom of the ladle 12, while the molten steel in the ladle 12 is blown from the powder injection lance from above. Oxidizer powder is blown in. The inside of the device is constantly evacuated and kept under reduced pressure.

FIG. 2 is a schematic explanatory view of an embodiment of the present invention in ladle refining using a dip tube. In the figure, a cylindrical dip tube 20 is placed at the bottom of molten steel 24 housed in a ladle 22. Is immersed, the inside of the immersion pipe is decompressed, and the stirring gas from the injection lance 26 is blown from below the immersion pipe. The stirring gas may be blown from the bottom blowing tuyere. A powder injection lance 28 is provided above the surface of the molten steel inside the dip tube, and the oxidant powder is injected.

FIG. 3 is a schematic explanatory view of an embodiment of the present invention using a reflux type degassing apparatus, in which the reflux type degassing apparatus is used.
30 is equipped with two dip pipes 32 and 34, and molten steel is circulated between them. The inside of the apparatus is decompressed, and an upper blowing lance 36 for blowing the oxidant powder is provided above the surface of the molten steel inside. One of the immersion pipes functions as an ascending pipe, and in the illustrated example, the immersion pipe 32 is an ascending pipe, and an inert gas (e.g., A
(gas) is blown in.

Further, the steel types to be treated in the present invention are all stainless steel and other carbon steels and alloy steels which are desired to have high quality and which are subjected to the refining treatment under reduced pressure as described above. . In the present specification, the terms "steel" and "molten steel" are used for convenience, but Fe
An alloy having a content of 50 wt% or less, for example, a Ni-based alloy is also included. Next, the operation and effect of the present invention will be described more specifically by way of examples.

[0023]

[Example] In a 50t VOD furnace as shown in FIG. 1, molten iron oxide powder was melted steel temperature: 1600 ° C Vacuum degree: 20Torr Powder particle size: 10 to 200 μm Powder supply rate: 0.6kg / min · t Lance -Distance between molten steel surfaces: A total of 2 kg / t was blown up at 0.5 m and 2 m. [C] of molten steel is 0.33%
Met.

Here, the bottom blown gas flow rate was set to 3 to 5 Nl / min-t with 2 pores. The results at this time are shown in Examples 1 to 6 in Table 1. In each case, it can be seen that denitrification is sufficiently performed. On the other hand, under the same conditions, the powder particle size should be <10.
Comparative examples 1 and 2 in Table 1 show the case where the thickness is 200 μm and 200 to 400 μm. As can be seen from the above, when the amount is out of the range of the present invention, sufficient denitrification was not performed and denitrification became insufficient.

Similarly, in a 50t VOD furnace as shown in FIG. 1, molten iron oxide powder was melted at a steel temperature: 1600 ° C. Vacuum degree: 20 Torr Powder particle size: 10 to 200 μm Powder supply rate: 0.08 kg / min t Lance-molten steel surface distance: A total of 8 kg / t was blown up at 1 m.

However, also in this case, denitrification was insufficient and the treatment time was long (see the results of Comparative Example 3).
. Under the same conditions, when only the powder supply rate was 2 kg / min · t, denitrification was insufficient (see the result of Comparative Example 4).

Next, in a 50 t VOD furnace, molten steel temperature: 1600 ° C. Vacuum degree: 20 Torr Powder particle size: 10 to 200 μm Powder supply rate: 1.0 kg / min t lance / molten steel surface distance: 0.3 m In this case, the lance was melted (Comparative Example 5). Under the same conditions, when the distance between the molten lance molten steel surfaces was set to 2.5 m, denitrification was insufficient, and it was observed that a large amount of powder flew up in the vacuum chamber (Comparative Example 6).

[0028]

[Table 1]

A cylindrical dip tube is immersed in molten steel in a ladle as illustrated in FIG. 2, and an inert gas is blown from a lance in a state where the dip tube is evacuated to a vacuum state.
At 250 ton ladle refining equipment, molten steel temperature: 1650 ℃ Vacuum degree: 3 Torr Bottom blowing stirring gas flow rate 10 Nl / min ・ t, powder particle size: 10-200 μm Powder supply rate: 0.5 kg / min・ The distance between t lance and molten steel surface: 0.5 m and 2 m, iron oxide powder was sprayed on 8 kg / ton in total. At this time,
The denitrification was performed well (Examples 7 to 12). On the other hand, when the distance between the lance and the molten steel surface was 0.3 m and 2.2 m, denitrification was insufficient (Comparative Examples 7 and 8).

[0030]

[Table 2]

In a 250 t reflux type degassing apparatus as shown in FIG. 3, molten steel temperature: 1650 ° C. Vacuum degree: 7 Torr reflux gas flow rate: 5-8 Nl / min · t, powder particle size: 10- 200 μm Powder injection speed: 1 kg / min · t lance / distance between molten steel surfaces: 1 m and 3 m, iron oxide powder was sprayed on a total of 8 kg / ton. At this time, denitrification was favorably performed (Examples 13 to 18). However, denitrification was insufficient when the distance between the lance molten steel surfaces was 0.8 m and 3.1 m (Comparative Examples 11 to 14).

[0032]

[Table 3]

[0033]

INDUSTRIAL APPLICABILITY According to the present invention, denitrification of molten steel is effectively performed by blowing a small amount of oxidizer powder such as [N] ≦ 10 ppm. In addition, according to the present invention, the effect of promoting desulfurization and decarburization reaction can be obtained at the same time, which is significant from a practical point of view.

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram of an implementation status of the present invention in a vacuum decarburization furnace (VOD furnace).

FIG. 2 is a schematic explanatory view of an implementation state of the present invention in ladle refining using an immersion tube.

FIG. 3 is a schematic explanatory diagram of an implementation state of the present invention using a reflux type degassing apparatus.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Ikemiya 3 Hikari No. 3, Kashima-cho, Kashima-gun, Kashima-gun, Ibaraki Sumitomo Metal Industries, Ltd. Kashima Steel Works (72) Akihiko Ebihara 4-53, Kitahama, Chuo-ku, Osaka Sumitomo Metal Industries Co., Ltd. (56) Reference JP 61-281807 (JP, A) JP 62-164816 (JP, A) JP 6-73429 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21C 7/10 C21C 7/00 C21C 7/072

Claims (4)

(57) [Claims] 1. In a refining method of decarburizing and denitrifying by spraying an oxidizer powder onto molten steel under reduced pressure, the oxidizer powder has a particle size of 10 to 200 μm and a powder supply rate. 0.1
~ 1.0kg / min · ton, Low nitrogen steel manufacturing method characterized by the following. 2. A ladle for containing molten steel is housed in a vacuum vessel, the inside of the vessel is evacuated and decompressed, and an inert gas is blown into the molten steel from the tuyere located at the bottom of the ladle. Of the oxidizer powder with a particle size of 10 to 200 μm from the powder injection lance with the distance from the molten steel surface of 0.5 to 2.0 m
A method for producing a low-nitrogen steel, characterized by carrying out decarburization and denitrification by top-spraying at ~ 1.0 kg / min · ton. 3. A cylindrical immersion pipe is immersed in molten steel in a ladle, the interior of the immersion pipe is exhausted and decompressed to suck the molten steel into the immersion pipe, and the immersion is performed from a lance located below the immersion pipe. The inert gas is blown toward the inside of the tube, and the oxidizer powder with a particle size of 10 to 200 μm is supplied from the powder injection lance with a distance of 0.5 to 2.0 m from the molten steel surface inside the immersion tube. 0.
Performs decarburization and denitrification by top-spraying at 1 to 1.0 kg / min ・ ton.
Method for producing a low nitrogen steel, characterized in that the Hare. 4. A reflux type degassing apparatus is immersed in molten steel in a ladle, and the molten steel is refluxed in the reflux type degassing apparatus,
The distance from the molten steel surface in the reflux type degasser is 1.0 to 3.0.
than a powder blowing lance is m the oxidant powder having a particle size of 10 to 200 [mu] m Shi blown up in the powder feed rate 0.1 ~1.0 kg / min · ton
A method for producing low-nitrogen steel, which comprises decarburizing and denitrifying by means of: